U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY. Robert L. Virta 1

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1 U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY A s b e s t o s : G e o l o g y, M i n e r a l o g y, M i n i n g, a n d U s e s b y Robert L. Virta 1 O p e n - F i l e R e p o r t Prepared in cooperation with Kirk-Othmer Encyclopedia of Chemical Technology, Online Edition, Wylie- Interscience, a division of John Wiley & Sons, Inc., New York, NY This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards (or with the North American Stratigraphic Code). Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. 1 Reston, VA

2 A b s t r a c t I n t r o d u c t i o n H i s t o r y Geology and Fiber Morphology Chrysotile A m p h i b o l e s C r y s t a l S t r u c t u r e o f A s b e s t o s F i b e r s Chrysotile A m p h i b o l e s P r o p e r t i e s o f A s b e s t o s F i b e r s F i b e r L e n g t h D i s t r i b u t i o n P h y s i c o - c h e m i c a l P r o p e r t i e s T h e r m a l B e h a v i o r T e n s i l e S t r e n g t h A s b e s t o s F i b e r s i n A q u e o u s M e d i a O t h e r B u l k P h y s i c a l P r o p e r t i e s S u r f a c e P r o p e r t i e s Surface Area S u r f a c e C h a r g e i n A q u e o u s M e d i a A d s o r p t i o n a n d S u r f a c e C h e m i c a l G r a f t i n g A n a l y t i c a l M e t h o d s a n d I d e n t i f i c a t i o n o f A s b e s t o s F i b e r s M i c r o s c o p i c M e t h o d s I n s t r u m e n t a l M e t h o d s f o r B u l k S a m p l e s P r o d u c t i o n M i n i n g a n d M i l l i n g T e c h n o l o g i e s F i b e r C l a s s i f i c a t i o n a n d S t a n d a r d T e s t i n g M e t h o d s Dry Classification Method W e t C l a s s i f i c a t i o n M e t h o d O t h e r F i b e r E v a l u a t i o n M e t h o d s I n d u s t r i a l A p p l i c a t i o n s A l t e r n a t i v e I n d u s t r i a l F i b e r s a n d M a t e r i a l s H e a l t h a n d S a f e t y R e g u l a t i o n B i b l i o g r a p h y T a b l e 1. A s b e s t o s F i b e r P r o d u c t i o n T a b l e 2. G e o l o g i c a l O c c u r r e n c e o f A s b e s t o s F i b e r s T a b l e 3. E l e m e n t a l A n a l y s i s o f A s b e s t o s F i b e r s T a b l e 4. P h y s i c a l a n d C h e m i c a l P r o p e r t i e s o f A s b e s t o s F i b e r s T a b l e 5. W o r l d P r o d u c t i o n o f A s b e s t o s b y P r i n c i p a l P r o d u c i n g C o u n t r i e s, a,b, t o n s

3 T a b l e 6. U t i l i z a t i o n o f A s b e s t o s F i b e r s b y P r o d u c t C a t e g o r y a n d P r o c e s s i n T a b l e 7. U s e s o f A s b e s t o s T a b l e 8. A s b e s t o s S u b s t i t u t e s a n d R e l a t i v e C o s t s Figures F i g u r e 1. A s b e s t o s f i b e r s ( c h r y s o t i l e, c r o c i d o l i t e, a n d a m o s i t e ) a s s e p a r a t e d f r o m h o s t r o c k a n d t h e i r m a s s i v e v a r i e t i e s ( a n t i g o r i t e, riebeckite, cummingtonite-grunerite) F i g u r e 2. E l e c t r o n m i c r o g r a p h s o f a s b e s t o s f i b e r s : ( a ) c h r y s o t i l e ; ( b ) c r o c i d o l i t e F i g u r e 3. S i l i c a t e f r a m e w o r k o f c h r y s o t i l e a n d a m p h i b o l e s : ( a ) t h e s h e e t s i l i c a t e s t r u c t u r e o f c h r y s o t i l e, a n a l o g o u s t o t h a t o f m i c a s ; ( b ) t h e d o u b l e - c h a i n s i l i c a t e s t r u c t u r e f o u n d i n a m p h i b o l e s F i g u r e 4. M i c r o s c o p i c s t r u c t u r e o f c h r y s o t i l e f i b e r s F i g u r e 5. M i c r o s c o p i c s t r u c t u r e o f a m p h i b o l e f i b e r s F i g u r e 6. F i b e r l e n g t h d i s t r i b u t i o n f o r ( a ) a l o n g s a m p l e ( g r o u p 4 ) a n d ( b ) a s h o r t s a m p l e ( g r o u p 7 ) o f c h r y s o t i l e ; s u c c e s s i v e l e n g t h classes separated by 50 :m F i g u r e 7. T h e r m a l a n a l y s i s c u r v e s f o r a s b e s t o s : ( a ) c h r y s o t i l e ; ( b ) c r o c i d o l i t e ( i n i n e r t a t m o s p h e r e ) F i g u r e 8. W o r l d p r o d u c t i o n o f a s b e s t o s, F i g u r e 9. S c h e m a t i c o f a t y p i c a l a s b e s t o s m i l l i n g f l o w l i n e ( 3 0 m e s h :m )

4 ABSTRACT T h e t e r m a s b e s t o s i s a g e n e r i c d e s i g n a t i o n r e f e r r i n g u s u a l l y t o s i x t y p e s o f n a t u r a l l y o c c u r r i n g m i n e r a l f i b e r s t h a t a r e o r h a v e b e e n c o m m e r c i a l l y e x p l o i t e d. T h e s e f i b e r s b e l o n g t o t w o m i n e r a l g r o u p s : s e r p e n t i n e s a n d a m p h i b o l e s. T h e s e r p e n t i n e g r o u p c o n t a i n s a s i n g l e a s b e s t i f o r m v a r i e t y : c h r y s o t i l e ; f i v e a s b e s t i f o r m v a r i e t i e s o f a m p h i b o l e s a r e k n o w n : a n t h o p h y l l i t e a s b e s t o s, g r u n e r i t e a s b e s t o s ( a m o s i t e ), r i e b e c k i t e a s b e s t o s ( c r o c i d o l i t e ), t r e m o l i t e a s b e s t o s, a n d a c t i n o l i t e a s b e s t o s. T h e s e f i b r o u s m i n e r a l s s h a r e s e v e r a l p r o p e r t i e s w h i c h q u a l i f y t h e m a s a s b e s t i f o r m f i b e r s : t h e y a r e f o u n d i n b u n d l e s o f f i b e r s w h i c h c a n b e e a s i l y s e p a r a t e d f r o m t h e h o s t m a t r i x o r c l e a v e d i n t o t h i n n e r f i b e r s ; t h e f i b e r s e x h i b i t h i g h t e n s i l e s t r e n g t h s, t h e y s h o w h i g h l e n g t h : d i a m e t e r ( a s p e c t ) r a t i o s, f r o m a m i n i m u m o f 2 0 u p t o g r e a t e r t h a n ; t h e y a r e s u f f i c i e n t l y f l e x i b l e t o b e s p u n ; a n d m a c r o s c o p i c a l l y, t h e y r e s e m b l e o r g a n i c f i b e r s s u c h a s c e l l u l o s e. S i n c e a s b e s t o s f i b e r s a r e a l l s i l i c a t e s, t h e y e x h i b i t s e v e r a l o t h e r c o m m o n p r o p e r t i e s, s u c h a s i n c o m b u s t i b i l i t y, t h e r m a l s t a b i l i t y, r e s i s t a n c e t o b i o d e g r a d a t i o n, c h e m i c a l i n e r t i a t o w a r d m o s t c h e m i c a l s, a n d l o w e l e c t r i c a l c o n d u c t i v i t y. T h e t e r m a s b e s t o s h a s t r a d i t i o n a l l y b e e n a t t r i b u t e d o n l y t o t h o s e v a r i e t i e s t h a t a r e c o m m e r c i a l l y e x p l o i t e d. T h e i n d u s t r i a l a p p l i c a t i o n s o f a s b e s t o s f i b e r s h a v e n o w s h i f t e d a l m o s t e x c l u s i v e l y t o c h r y s o t i l e. T w o t y p e s o f a m p h i b o l e s, c o m m o n l y d e s i g n a t e d a s a m o s i t e a n d c r o c i d o l i t e a r e n o l o n g e r m i n e d. T h e o t h e r t h r e e a m p h i b o l e v a r i e t i e s, a n t h o p h y l l i t e a s b e s t o s, a c t i n o l i t e a s b e s t o s, a n d t r e m o l i t e a s b e s t o s, h a v e n o s i g n i f i c a n t i n d u s t r i a l a p p l i c a t i o n s p r e s e n t l y. T h e m i c r o s c o p i c a n d m a c r o s c o p i c p r o p e r t i e s o f a s b e s t o s f i b e r s s t e m f r o m t h e i r i n t r i n s i c, a n d s o m e t i m e s u n i q u e, c r y s t a l l i n e f e a t u r e s. A s w i t h a l l s i l i c a t e m i n e r a l s, t h e b a s i c b u i l d i n g b l o c k s o f a s b e s t o s f i b e r s a r e t h e s i l i c a t e t e t r a h e d r a w h i c h m a y o c c u r a s d o u b l e c h a i n s (Si 4 O 11 ) -6, a s i n t h e a m p h i b o l e s, o r i n s h e e t s ( S i 4 O 10 ) -4, as in chrysotile. In the case of chrysotile, an octahedral brucite layer having the formula (Mg 6 O 4 (OH) 8 ) -4 i s i n t e r c a l a t e d b e t w e e n e a c h s i l i c a t e t e t r a h e d r a s h e e t. A s b e s t o s f i b e r s u s e d i n m o s t i n d u s t r i a l a p p l i c a t i o n s c o n s i s t o f a g g r e g a t e s o f s m a l l e r u n i t s ( f i b r i l s ). T h i s i s m o s t e v i d e n t w i t h c h r y s o t i l e t h a t e x h i b i t s a n i n h e r e n t, w e l l - d e f i n e d u n i t f i b e r. T h e i d e n t i f i c a t i o n o f a s b e s t o s f i b e r s c a n b e p e r f o r m e d t h r o u g h m o r p h o l o g i c a l e x a m i n a t i o n, t o g e t h e r w i t h s p e c i f i c a n a l y t i c a l m e t h o d s t o o b t a i n t h e m i n e r a l c o m p o s i t i o n a n d / o r s t r u c t u r e. M o r p h o l o g i c a l c h a r a c t e r i z a t i o n i n i t s e l f u s u a l l y d o e s n o t c o n s t i t u t e a r e l i a b l e i d e n t i f i c a t i o n c r i t e r i o n. H e n c e, m i c r o s c o p i c e x a m i n a t i o n m e t h o d s a n d o t h e r a n a l y t i c a l a p p r o a c h e s a r e u s u a l l y c o m b i n e d. M o s t o f t h e a s b e s t o s m i n i n g o p e r a t i o n s a r e o f t h e o p e n p i t t y p e, u s i n g b e n c h d r i l l i n g t e c h n i q u e s. T h e f i b e r e x t r a c t i o n ( m i l l i n g ) p r o c e s s m u s t b e c h o s e n s o a s t o o p t i m i z e r e c o v e r y o f t h e f i b e r s i n t h e o r e, w h i l e m i n i m i z i n g r e d u c t i o n o f f i b e r l e n g t h. D r y m i l l i n g o p e r a t i o n s a r e t h e m o s t w i d e l y u s e d. I n t h e p r o d u c t i o n, o r i n d u s t r i a l a p p l i c a t i o n s, o f a s b e s t o s f i b e r s, s e v e r a l p a r a m e t e r s a r e c o n s i d e r e d c r i t i c a l l y i m p o r t a n t. T h e m e a s u r e m e n t o f f i b e r l e n g t h i s i m p o r t a n t s i n c e t h e l e n g t h d e t e r m i n e s t h e p r o d u c t c a t e g o r y i n w h i c h t h e f i b e r s w i l l b e u s e d a n d, t o a l a r g e e x t e n t, t h e i r c o m m e r c i a l v a l u e. T h e m o s t w i d e l y a c c e p t e d m e t h o d f o r c h r y s o t i l e f i b e r l e n g t h c h a r a c t e r i z a t i o n i n t h e i n d u s t r y i s t h e Q u e b e c S t a n d a r d t e s t. A s e c o n d i n d u s t r i a l l y i m p o r t a n t f i b e r - l e n g t h e v a l u a t i o n t e c h n i q u e i s t h e B a u e r - M c N e t t c l a s s i f i c a t i o n. A s b e s t o s f i b e r s h i s t o r i c a l l y h a v e b e e n u s e d i n a b r o a d v a r i e t y o f i n d u s t r i a l a p p l i c a t i o n s. B e c a u s e o f r e c e n t r e s t r i c t i o n s, m a n y o f t h e s e a p p l i c a t i o n s h a v e n o w b e e n a b a n d o n e d a n d o t h e r s a r e p u r s u e d u n d e r s t r i c t l y r e g u l a t e d c o n d i t i o n s. T h e m a i n c h a r a c t e r i s t i c p r o p e r t i e s o f a s b e s t o s f i b e r s t h a t c a n b e e x p l o i t e d i n i n d u s t r i a l a p p l i c a t i o n s a r e t h e i r t h e r m a l, e l e c t r i c a l, a n d s o u n d i n s u l a t i o n ; i n f l a m m a b i l i t y ; m a t r i x r e i n f o r c e m e n t ( c e m e n t, p l a s t i c, a n d r e s i n s ) ; a d s o r p t i o n c a p a c i t y ( f i l t r a t i o n, l i q u i d s t e r i l i z a t i o n ) ; w e a r a n d f r i c t i o n p r o p e r t i e s ( f r i c t i o n m a t e r i a l s ) ; a n d c h e m i c a l i n e r t i a ( e x c e p t i n a c i d s ). T h e s e p r o p e r t i e s l e d t o s e v e r a l m a i n c l a s s e s o f i n d u s t r i a l p r o d u c t s o r a p p l i c a t i o n s : f i r e p r o t e c t i o n a n d h e a t o r s o u n d i n s u l a t i o n, f a b r i c a t i o n o f p a p e r s a n d f e l t s f o r f l o o r i n g a n d r o o f i n g p r o d u c t s, p i p e l i n e w r a p p i n g, e l e c t r i c a l i n s u l a t i o n, t h e r m a l a n d e l e c t r i c a l i n s u l a t i o n, f r i c t i o n p r o d u c t s i n b r a k e o r c l u t c h p a d s, a s b e s t o s - c e m e n t p r o d u c t s, r e i n f o r c e m e n t o f p l a s t i c s, f a b r i c a t i o n o f p a c k i n g s a n d g a s k e t s, f r i c t i o n m a t e r i a l s f o r b r a k e l i n i n g s a n d p a d s, r e i n f o r c i n g a g e n t s, v i n y l o r a s p h a l t t i l e s, a n d a s p h a l t r o a d s u r f a c i n g. O f t h e s e, a s b e s t o s - c e m e n t p r o d u c t s, r o o f c o a t i n g s, b r a k e p a d s a n d s h o e s, a n d c l u t c h e s a r e t h e m a j o r m a r k e t s f o r a s b e s t o s. T h e r e l a t i o n s h i p b e t w e e n w o r k p l a c e e x p o s u r e t o a i r b o r n e a s b e s t o s f i b e r s a n d r e s p i r a t o r y d i s e a s e s i s o n e o f t h e m o s t w i d e l y s t u d i e d s u b j e c t s o f m o d e r n e p i d e m i o l o g y. T h e r e s e a r c h e f f o r t s r e s u l t e d i n s i g n i f i c a n t c o n s e n s u s i n s o m e a r e a s, a l t h o u g h s t r o n g c o n t r o v e r s i e s r e m a i n i n o t h e r a r e a s. T y p i c a l l y, i t i s w i d e l y r e c o g n i z e d t h a t t h e i n h a l a t i o n o f l o n g ( c o n s i d e r e d u s u a l l y a s l e s s t h a n 5 m i c r o m e t e r s [:m ] ), t h i n, a n d d u r a b l e f i b e r s c a n i n d u c e o r p r o m o t e l u n g c a n c e r. I t i s a l s o w i d e l y a c c e p t e d t h a t a s b e s t o s f i b e r s c a n b e a s s o c i a t e d w i t h t h r e e t y p e s o f d i s e a s e s : a s b e s t o s i s, l u n g c a n c e r, m e s o t h e l i o m a. A f u r t h e r c o n s e n s u s d e v e l o p e d w i t h i n t h e s c i e n t i f i c c o m m u n i t y r e g a r d i n g t h e r e l a t i v e c a r c i n o g e n i c i t y o f t h e d i f f e r e n t t y p e s o f a s b e s t o s f i b e r s. T h e r e i s s t r o n g e v i d e n c e t h a t t h e g e n o t o x i c a n d c a r c i n o g e n i c p o t e n t i a l s o f a s b e s t o s f i b e r s a r e n o t i d e n t i c a l ; i n p a r t i c u l a r, m e s o t h e l i a l c a n c e r i s m o s t l y a s s o c i a t e d w i t h a m p h i b o l e f i b e r s. T h e i d e n t i f i c a t i o n o f h e a l t h r i s k s a s s o c i a t e d w i t h a s b e s t o s f i b e r s h a s p r o m p t e d s t r i c t r e g u l a t i o n s t o l i m i t t h e m a x i m u m e x p o s u r e o f a i r b o r n e f i b e r s i n w o r k p l a c e e n v i r o n m e n t s. 4

5 1. INTRODUCTION Asbestos is a generic term referring to six types of naturally occurring mineral fibers that are or have been commercially exploited. T h e s e f i b e r s b e l o n g t o t w o m i n e r a l g r o u p s : s e r p e n t i n e s a n d a m p h i b o l e s. T h e s e r p e n t i n e g r o u p c o n t a i n s a s i n g l e a s b e s t i f o r m v a r i e t y : c h r y s o t i l e. T h e r e a r e f i v e a s b e s t i f o r m v a r i e t i e s o f a m p h i b o l e s : a n t h o p h y l l i t e a s b e s t o s, g r u n e r i t e a s b e s t o s ( a m o s i t e ), r i e b e c k i t e a s b e s t o s ( c r o c i d o l i t e ), t r e m o l i t e a s b e s t o s, a n d a c t i n o l i t e a s b e s t o s. U s u a l l y, t h e t e r m a s b e s t o s i s a p p l i e d o n l y t o t h o s e v a r i e t i e s t h a t h a v e b e e n c o m m e r c i a l l y e x p l o i t e d ( 1, 2 ). T h a t d o e s n o t p r e c l u d e t h e o c c u r r e n c e o f o t h e r a s b e s t o s - l i k e m i n e r a l s, h o w e v e r. M a g n e s i o r i e b e c k i t e w i t h a n a s b e s t i f o r m h a b i t w a s m i n e d i n B o l i v i a a n d p o t a s s i a n w i n c h i t e w i t h a n a s b e s t i f o r m h a b i t w a s f o u n d i n w e s t e r n T e x a s. A d d i t i o n a l l y, r i c h t e r i t e a s b e s t o s h a s b e e n s y n t h e s i z e d i n t h e l a b o r a t o r y ( 3 ). T h e a s b e s t o s v a r i e t i e s s h a r e s e v e r a l p r o p e r t i e s : (1 ) t h e y o c c u r a s b u n d l e s o f f i b e r s t h a t c a n b e e a s i l y s e p a r a t e d f r o m t h e h o s t m a t r i x o r c l e a v e d i n t o t h i n n e r f i b e r s ( 1, 4 ) ; (2 ) t h e f i b e r s e x h i b i t h i g h t e n s i l e s t r e n g t h s ( 1 ) ; (3 ) t h e y s h o w h i g h l e n g t h : d i a m e t e r ( a s p e c t ) r a t i o s, w i t h a m i n i m u m o f 2 0 a n d u p t o ( 1, 4 ) ; (4 ) they are sufficiently flexible to be spun; and (5 ) macroscopically, they resemble o r g a n i c f i b e r s s u c h a s c e l l u l o s e ( 2, 4 ). S i n c e a s b e s t o s f i b e r s a r e a l l s i l i c a t e s, t h e y e x h i b i t s e v e r a l o t h e r c o m m o n p r o p e r t i e s, s u c h a s i n c o m b u s t i b i l i t y, t h e r m a l s t a b i l i t y, r e s i s t a n c e t o b i o d e g r a d a t i o n, c h e m i c a l i n e r t i a t o w a r d m o s t c h e m i c a l s, a n d l o w e l e c t r i c a l c o n d u c t i v i t y. T h e u s u a l d e f i n i t i o n o f a s b e s t o s f i b e r e x c l u d e s n u m e r o u s o t h e r f i b r o u s m i n e r a l s t h a t m a y p o s s e s s a n a s b e s t i f o r m h a b i t b u t d o n o t e x h i b i t a l l o f t h e p r o p e r t i e s o f a s b e s t o s. A f e w e x a m p l e s o f t h e s e f i b r o u s m i n e r a l s a r e s e p i o l i t e, e r i o n i t e ( r o d - l i k e a n d f i b r o u s h a b i t s ), a n d n e m o l i t e. O t h e r m i n e r a l s a l s o m a y o c c a s i o n a l l y c r y s t a l l i z e w i t h a f i b r o u s h a b i t ( 3, 4 ). T h e m i n e r a l o g i c a l d e s i g n a t i o n s o f t h e v a r i o u s a s b e s t o s f i b e r s, t h e i r m o s t c o m m o n a l t e r n a t i v e d e s i g n a t i o n s, a n d t h e m a i n s o u r c e s o f t h e s e f i b e r s a r e r e p o r t e d i n T a b l e 1. T h e f r a c t i o n a l b r e a k d o w n o f t h e r e c e n t w o r l d p r o d u c t i o n o f t h e v a r i o u s f i b e r t y p e s s h o w s t h a t t h e i n d u s t r i a l a p p l i c a t i o n s o f a s b e s t o s f i b e r s h a v e n o w s h i f t e d a l m o s t e x c l u s i v e l y t o c h r y s o t i l e. A m o s i t e a n d c r o c i d o l i t e a r e n o l o n g e r b e i n g m i n e d a l t h o u g h s o m e p r o b a b l y i s s t i l l b e i n g s o l d f r o m s t o c k. C u r r e n t u s e o f a m o s i t e a n d c r o c i d o l i t e i s e s t i m a t e d t o b e l e s s t h a n a f e w h u n d r e d m e t r i c t o n s a n n u a l l y. A c t i n o l i t e a s b e s t o s, a n t h o p h y l l i t e a s b e s t o s, a n d t r e m o l i t e a s b e s t o s m a y b e s t i l l m i n e d i n s m a l l a m o u n t s f o r l o c a l u s e ; p r o d u c t i o n p r o b a b l y i s l e s s t h a n m e t r i c t o n s a n n u a l l y. 2. HISTORY E a r l y u s e s o f a s b e s t o s e x p l o i t e d t h e r e i n f o r c e m e n t a n d t h e r m a l p r o p e r t i e s o f a s b e s t o s f i b e r s. T h e f i r s t r e c o r d e d a p p l i c a t i o n c a n b e t r a c e d t o F i n l a n d ( a p p r o x i m a t e l y B. C. ), w h e r e a n t h o p h y l l i t e f r o m a l o c a l d e p o s i t w a s u s e d t o r e i n f o r c e c l a y u t e n s i l s a n d p o t t e r y ( 5 ). N u m e r o u s e a r l y r e f e r e n c e s a l s o c a n b e f o u n d d e s c r i b i n g t h e u s e o f a s b e s t o s f i b e r s f o r t h e f a b r i c a t i o n o f l a m p w i c k s a n d c r e m a t o r y c l o t h i n g. O t h e r a p p l i c a t i o n s o f a s b e s t o s f i b e r s i n h e a t - o r f l a m e - r e s i s t a n t m a t e r i a l s h a v e b e e n s p o r a d i c a l l y r e p o r t e d. A t t h e e n d o f t h e s e v e n t e e n t h c e n t u r y, P e t e r t h e G r e a t o f R u s s i a i n i t i a t e d t h e f a b r i c a t i o n o f a s b e s t o s p a p e r, u s i n g c h r y s o t i l e f i b e r s e x t r a c t e d f r o m d e p o s i t s i n t h e U r a l M o u n t a i n s. T h e u s e o f a s b e s t o s f i b e r s o n a t r u e i n d u s t r i a l s c a l e b e g a n i n I t a l y e a r l y i n t h e n i n e t e e n t h c e n t u r y w i t h t h e d e v e l o p m e n t o f a s b e s t o s t e x t i l e s ( 4, 6 ). B y t h e e n d o f t h e n i n e t e e n t h c e n t u r y, s i g n i f i c a n t a s b e s t o s d e p o s i t s h a d b e e n i d e n t i f i e d t h r o u g h o u t t h e w o r l d a n d t h e i r e x p l o i t a t i o n h a d b e g u n i n C a n a d a ( ), S o u t h A f r i c a ( , ), a n d t h e U S S R ( ) ( 7 ). F r o m t h e b e g i n n i n g o f t h e 2 0 t h c e n t u r y, t h e d e m a n d f o r a s b e s t o s f i b e r s g r e w i n a s p e c t a c u l a r f a s h i o n f o r n u m e r o u s a p p l i c a t i o n s, i n p a r t i c u l a r f o r t h e r m a l i n s u l a t i o n ( 8 ). T h e d e v e l o p m e n t o f t h e H a t s c h e k m a c h i n e i n f o r t h e c o n t i n u o u s f a b r i c a t i o n o f s h e e t s f r o m a n a s b e s t o s - c e m e n t c o m p o s i t e a l s o o p e n e d a n i m p o r t a n t f i e l d o f i n d u s t r i a l a p p l i c a t i o n f o r a s b e s t o s f i b e r s a s d i d t h e d e v e l o p m e n t o f t h e a u t o m o b i l e i n d u s t r y f o r a s b e s t o s b r a k e s, c l u t c h e s, a n d g a s k e t s. W o r l d W a r I I s u p p o r t e d t h e g r o w t h o f a s b e s t o s f i b e r p r o d u c t i o n f o r m i l i t a r y a p p l i c a t i o n s, t y p i c a l l y i n t h e r m a l i n s u l a t i o n a n d f i r e p r o t e c t i o n. S u c h a p p l i c a t i o n s w e r e l a t e r e x t e n d e d i n t o r e s i d e n t i a l o r i n d u s t r i a l c o n s t r u c t i o n s f o r s e v e r a l d e c a d e s f o l l o w i n g t h e w a r. D u r i n g t h e l a t e s a n d s, t h e f i n d i n g o f h e a l t h p r o b l e m s a s s o c i a t e d w i t h l o n g - t e r m h e a v y e x p o s u r e t o a i r b o r n e a s b e s t o s f i b e r s l e d t o a l a r g e r e d u c t i o n i n t h e u s e o f a s b e s t o s f i b e r s. I n m o s t o f t h e c u r r e n t a p p l i c a t i o n s, a s b e s t o s f i b e r s a r e c o n t a i n e d w i t h i n a matrix, typically cement or organic resins. T h e w o r l d p r o d u c t i o n o f a s b e s t o s f i b e r s r e a c h e d a m a x i m u m i n o f 4. 8 x t o n s, d e c r e a s i n g t o 1. 9 x t o n s i n T h e m a j o r p r o d u c i n g c o u n t r i e s o f c h r y s o t i l e a s b e s t o s a r e R u s s i a ( 3 9 % ), C a n a d a ( 1 8 % ), C h i n a ( 1 4 % ), B r a z i l ( 9 % ), K a z a k h s t a n ( 7 % ), a n d Z i m b a b w e ( 6 % ). I n , a c t i v e m i n i n g o p e r a t i o n s o f a s b e s t o s f i b e r s a r e f o u n d i n 2 1 c o u n t r i e s ( 9 ). 3. GEOLOGY AND FIBER MORPHOLOGY T h e g e n e s i s o f a s b e s t o s f i b e r s a s m i n e r a l d e p o s i t s r e q u i r e d c e r t a i n c o n d i t i o n s w i t h r e g a r d t o c h e m i c a l c o m p o s i t i o n, n u c l e a t i o n, a n d f i b e r g r o w t h ; s u c h c o n d i t i o n s m u s t h a v e p r e v a i l e d o v e r a p e r i o d s u f f i c i e n t l y l o n g a n d p e r t u r b a t i o n - f r e e t o a l l o w a c o n t i n u o u s g r o w t h o f t h e s i l i c a t e c h a i n s i n t o f i b r o u s s t r u c t u r e s ( 1 0 ). S o m e o f t h e i m p o r t a n t g e o l o g i c a l o r m i n e r a l o g i c a l f e a t u r e s o f t h e i n d u s t r i a l l y s i g n i f i c a n t a s b e s t o s f i b e r s a r e s u m m a r i z e d i n T a b l e 2. M o r e e m p h a s i s i s g i v e n t o c h r y s o t i l e i n t h e f o l l o w i n g s e c t i o n o w i n g t o i t s t o t a l d o m i n a n c e in the industry over the years. O n l y t h r e e v a r i e t i e s o f a m p h i b o l e f i b e r s w i l l b e d i s c u s s e d b e c a u s e : (1 ) c r o c i d o l i t e a n d a m o s i t e w e r e t h e o n l y a m p h i b o l e s w i t h s i g n i f i c a n t i n d u s t r i a l u s e s i n r e c e n t y e a r s ; (2 ) t r e m o l i t e, a l t h o u g h h a v i n g e s s e n t i a l l y n o i n d u s t r i a l a p p l i c a t i o n, m a y b e f o u n d a s a c o n t a m i n a n t i n o t h e r f i b e r s o r i n o t h e r i n d u s t r i a l m i n e r a l s ( e. g., c h r y s o t i l e a n d t a l c ). 5

6 3. 1 C h r y s o t i l e C h r y s o t i l e b e l o n g s t o t h e s e r p e n t i n e g r o u p o f m i n e r a l s, v a r i e t i e s o f w h i c h a r e f o u n d i n u l t r a b a s i c r o c k f o r m a t i o n s l o c a t e d i n m a n y p l a c e s i n t h e w o r l d ( 1 1 ). C h r y s o t i l e a c c o u n t s f o r o n l y a s m a l l p e r c e n t a g e o f t h e m i n e r a l s f o u n d i n t h e s e r o c k t y p e s. C h r y s o t i l e f i b e r s a r e f o u n d a s v e i n s i n s e r p e n t i n e s, i n s e r p e n t i n i z e d u l t r a m a f i c r o c k s, a n d i n s e r p e n t i n i z e d d o l o m i t i c m a r b l e s. I t h a s b e e n s u g g e s t e d t h a t the ultrabasic rocks, containing olivine, Mg-rich pyroxenes, and amphiboles are first altered by hydrothermal processes to form the serpentine minerals; in a later metamorphic event, the serpentines are partially redissolved and crystallized as chrysotile fibers. Clearly, t h e g e n e s i s o f e a c h c h r y s o t i l e d e p o s i t m u s t h a v e i n v o l v e d s p e c i f i c f e a t u r e s r e l a t e d t o t h e c o m p o s i t i o n o f t h e p r e c u r s o r m i n e r a l s, t h e s t r e s s a n d d e f o r m a t i o n s i n t h e h o s t m a t r i x, t h e w a t e r c o n t e n t, t h e t e m p e r a t u r e c y c l e s, e t c. N o n e t h e l e s s, i t i s g e n e r a l l y o b s e r v e d t h a t t h e c h e m i c a l c o m p o s i t i o n o f t h e f i b r o u s p h a s e i s c l o s e l y r e l a t e d t o t h a t o f t h e s u r r o u n d i n g r o c k m a t r i x ( 1 2 ). G r o w t h o f c h r y s o t i l e f i b e r s a t r i g h t a n g l e s t o t h e w a l l s o f c r a c k s ( c r o s s - v e i n ) i n m a s s i v e s e r p e n t i n e f o r m a t i o n s l e d t o t h e m o s t c o m m o n t y p e o f c h r y s o t i l e d e p o s i t. M o s t o f t h e i n d u s t r i a l c h r y s o t i l e f i b e r s a r e e x t r a c t e d f r o m d e p o s i t s w h e r e f i b e r l e n g t h s c a n r e a c h s e v e r a l c e n t i m e t e r s ( c m ), b u t m o s t o f t e n d o n o t e x c e e d 1 c m ( 1 2 ). F i g u r e 1 i l l u s t r a t e s t h e t y p i c a l a s p e c t o f c r o s s - v e i n c h r y s o t i l e f i b e r s ( a l s o c a l l e d c r o s s f i b e r ) s e p a r a t e d f r o m t h e h o s t s e r p e n t i n e r o c k ; a n o n f i b r o u s v a r i e t y o f s e r p e n t i n e, a n t i g o r i t e ( [ ] ; C h e m i c a l A b s t r a c t S e r v i c e R e g i s t r y n u m b e r i n s q u a r e b r a c k e t s ), h a v i n g a c h e m i c a l c o m p o s i t i o n n e a r l y i d e n t i c a l t o t h a t o f c h r y s o t i l e, i s a l s o p i c t u r e d i n F i g u r e 1. I n s o m e o c c u r r e n c e s, t h e r e l a t i v e m o t i o n s ( s l i p p i n g ) o f b l o c k s i n t h e h o s t r o c k, d u r i n g o r a f t e r f i b e r g r o w t h, l e a d t o v e i n s i n w h i c h t h e f i b e r s a r e i n c l i n e d o r p a r a l l e l t o t h e v e i n a x e s ( s l i p f i b e r s ). I n s t i l l o t h e r l o c a l c o n d i t i o n s, d i s p e r s e d a g g r e g a t e s o f s h o r t f i b e r s a r e f o u n d w i t h n o p r e f e r e n t i a l o r i e n t a t i o n ; i n s u c h c a s e s t h e f i b e r c o n t e n t o f t h e r o c k c a n b e v e r y h i g h, u p t o 5 0 %. T h e s e a r e c a l l e d m a s s - f i b e r d e p o s i t s. C h r y s o t i l e i s a h y d r a t e d m a g n e s i u m s i l i c a t e a n d i t s s t o i c h i o m e t r i c c h e m i c a l c o m p o s i t i o n m a y b e g i v e n a s M g 3 Si 2 O 5 (OH) 4 [ ]. H o w e v e r, t h e g e o t h e r m a l p r o c e s s e s t h a t y i e l d t h e c h r y s o t i l e f i b e r f o r m a t i o n s u s u a l l y i n v o l v e t h e c o - d e p o s i t i o n o f v a r i o u s o t h e r m i n e r a l s. T h e s e m i n e r a l c o n t a m i n a n t s c o m p r i s e : b r u c i t e [ ] M g ( O H ) 2 ), magnetite [ ] (Fe 3 O 4 ), c a l c i t e [ ] (CaCO 3 ), dolomite [ ] ((Mg,Ca)(CO 3) 2 ), chlorite ((Mg,Al,Fe) 12 Si 8 O 20 (OH) 16 ), a n d t a l c [ ] ( M g 6 Si 8 O 20 (OH) 4 ). Other iron-containing minerals may also be found in chrysotile, for example, pyroaurite, brugnatellite, and pyroxenes. In commercial f i b e r s, d u s t p a r t i c l e s f r o m t h e h o s t r o c k g e n e r a t e d d u r i n g t h e m i n i n g a n d m i l l i n g p r o c e s s e s a r e m o s t i n e v i t a b l y p r e s e n t ( 3 ). E l e m e n t a l a n a l y s i s d a t a f o r s e v e r a l c h r y s o t i l e a n d a m p h i b o l e s a m p l e s a r e p r e s e n t e d i n T a b l e 3. Chrysotile fibers can be extremely thin, the unit fiber having an average diameter of approximately 25 nanometers (nm) (0.025 :m). I n d u s t r i a l c h r y s o t i l e f i b e r s a r e a g g r e g a t e s o f t h e s e u n i t f i b e r s t h a t u s u a l l y e x h i b i t d i a m e t e r s f r o m 0. 1 t o :m ; their lengths range f r o m a f r a c t i o n o f a m i l l i m e t e r ( m m ) t o s e v e r a l c e n t i m e t e r s ( c m ), t h o u g h m o s t o f t h e c h r y s o t i l e f i b e r s u s e d a r e s h o r t e r t h a n 1 c m. 3.2 Amphiboles T h e a m p h i b o l e g r o u p o f m i n e r a l s i s w i d e l y f o u n d t h r o u g h o u t t h e e a r t h ' s c r u s t. T h e i r c h e m i c a l c o m p o s i t i o n c a n v a r y w i d e l y. O f t h e a m p h i b o l e s, o n l y a f e w v a r i e t i e s h a v e a n a s b e s t i f o r m h a b i t a n d t h e l a t t e r o c c u r i n r e l a t i v e l y l o w q u a n t i t i e s. T h e g e o l o g i c a l o r i g i n o f a m p h i b o l e a s b e s t o s f i b e r s a p p e a r s t o b e q u i t e v a r i e d ( 3 ). I n t h e c a s e o f t h e c r o c i d o l i t e d e p o s i t o f S o u t h A f r i c a ( T r a n s v a a l ), t h e a m p h i b o l e f i b e r s f o r m e d d u r i n g s e c o n d a r y c h e m i c a l r e a c t i o n s t h a t t o o k p l a c e a s t h e b a n d e d i r o n s t o n e h o s t r o c k c o n s o l i d a t e d f r o m a g e l o f i r o n h y d r o x i d e a n d c o l l o i d a l s i l i c a. C r o c i d o l i t e f i b e r v e i n s f o r m e d, p r e s u m a b l y w i t h m a g n e t i t e p a r t i c l e s a c t i n g a s n u c l e a t i n g a g e n t s. T h e p r e s e n c e o f m e c h a n i c a l s t r e s s a p p e a r s t o b e n e c e s s a r y f o r t h e f o r m a t i o n o f c r o c i d o l i t e. I t a p p e a r s t h a t f i b e r s f o r m i n p l a c e s w h e r e t h e r e i s s h e a r i n g ( s l i p f i b e r s ) o r r o c k d i a l a t i u m ( c r o s s f i b e r ). O n e w o u l d n o t e x p e c t f i b e r s t o f o r m i n r o c k t h a t i s n o t m e c h a n i c a l l y d i s t u r b e d. T h e a m o s i t e d e p o s i t f o u n d i n s i m i l a r r o c k f o r m a t i o n s i s t h e r e s u l t o f a h i g h t e m p e r a t u r e m e t a m o r p h i c p r o c e s s ( 3 ). The chemical composition of amphiboles readily reflects the complexity of the environment in which they formed. The average c h e m i c a l c o m p o s i t i o n o f a m p h i b o l e m i n e r a l s m a y b e r e p r e s e n t e d a s : A 0-1, B 2 C 5 T 8 O 22 (OH, O, F, Cl) 2 where A= Na, K B= Na, Ca, Mg, Fe +2, Mn, Li C= Al, Fe +2, Fe +3, Ti, Mg, Mn, Cr T= Si, Al A, B, C e a c h r e p r e s e n t c a t i o n i c s i t e s w i t h i n t h e c r y s t a l s t r u c t u r e ( 2, 4, 1 0 ). F r o m t h i s g e n e r i c r e p r e s e n t a t i o n, t h e c h e m i c a l c o m p o s i t i o n o f t h e a m p h i b o l e a s b e s t o s c a n b e g i v e n a s f o l l o w s : 6

7 Name C o m p o s i t i o n Chemical Abstract Service Registry Number g r u n e r i t e ( a m o s i t e ) (Fe +2 ) 2 (Fe +2,Mg) 5 Si 8 O 22 (OH) 2 [ ] r i e b e c k i t e ( c r o c i d o l i t e ) Na 2 (Fe +2,Mg) 3 Fe +3 Si 8 O 22 (OH) 2 [ ] a n t h o p h y l l i t e Mg 7 Si 8 O 22 (OH) 2 [ ] t r e m o l i t e Ca 2 Mg 5 Si 8 O 22 (OH) 2 [ ] a c t i n o l i t e Ca 2 (Mg,Fe +2 )Si 8 O 22 (OH) 2 [ ] F r o m t h e i r r e s p e c t i v e c o m p o s i t i o n s, t h e a m p h i b o l e f i b e r s c a n b e v i e w e d a s a s e r i e s o f m i n e r a l s i n w h i c h o n e c a t i o n i s p r o g r e s s i v e l y r e p l a c e d b y a n o t h e r a t a g i v e n s i t e. F o r e x a m p l e, t r e m o l i t e a n d a c t i n o l i t e m a y b e s e e n r e l a t e d ; t h e m a g n e s i u m i n t r e m o l i t e i s p a r t l y r e p l a c e d b y d i v a l e n t i r o n i n t h e C p o s i t i o n t o y i e l d a c t i n o l i t e. S i m i l a r l y, i n t h e c u m m i n g t o n i t e - g r u n e r i t e a m p h i b o l e s, t h e s u b s t i t u t i o n i n v o l v e s t h e r e p l a c e m e n t o f m a g n e s i u m i n p o s i t i o n s B a n d C b y d i v a l e n t i r o n ( 2, 4 ). T h e c h a n g e o f d e s i g n a t i o n f r o m c u m m i n g t o n i t e t o grunerite is, by convention, at a Mg:Fe ratio of 0.5 (10). T h e t w o m o s t i m p o r t a n t a m p h i b o l e a s b e s t o s m i n e r a l s a r e a m o s i t e a n d c r o c i d o l i t e, a n d b o t h a r e h y d r a t e d s i l i c a t e s o f i r o n, m a g n e s i u m, a n d s o d i u m ( c r o c i d o l i t e o n l y ). T h e a p p e a r a n c e o f t h e s e f i b e r s a n d o f t h e c o r r e s p o n d i n g n o n f i b r o u s a m p h i b o l e s i s s h o w n i n F i g u r e 1. A l t h o u g h t h e m a c r o s c o p i c v i s u a l a s p e c t o f c l u s t e r s o f v a r i o u s t y p e s o f a s b e s t o s f i b e r s i s s i m i l a r, s i g n i f i c a n t d i f f e r e n c e s b e t w e e n c h r y s o t i l e a n d a m p h i b o l e s a p p e a r a t t h e m i c r o s c o p i c l e v e l. U n d e r t h e e l e c t r o n m i c r o s c o p e, c h r y s o t i l e f i b e r s a r e s e e n a s c l u s t e r s o f f i b r i l s, o f t e n e n t a n g l e d, s u g g e s t i n g l o o s e l y b o n d e d, f l e x i b l e f i b r i l s ( F i g. 2 a ). A m p h i b o l e f i b e r s, o n t h e o t h e r h a n d, o f t e n a p p e a r a s i n d i v i d u a l l y, r a t h e r t h a n i n f i b e r b u n d l e s ( F i g. 2 b ). 4. CRYSTAL STRUCTURE OF ASBESTOS FIBERS T h e m i c r o s c o p i c a n d m a c r o s c o p i c p r o p e r t i e s o f a s b e s t o s f i b e r s s t e m f r o m t h e i r i n t r i n s i c, a n d s o m e t i m e s u n i q u e, c r y s t a l l i n e f e a t u r e s. A s w i t h a l l s i l i c a t e m i n e r a l s, t h e b a s i c b u i l d i n g b l o c k s o f a s b e s t o s f i b e r s a r e t h e s i l i c a t e t e t r a h e d r a w h i c h m a y o c c u r a s d o u b l e c h a i n s (Si 4 O 11 ) -6, a s i n t h e a m p h i b o l e s, o r i n s h e e t s ( S i 4 O 10 ) -4, as in chrysotile (4) (Fig. 3) C h r y s o t i l e In the case of chrysotile, an octahedral brucite layer having the formula (Mg 6 O 4 (OH) 8) -4 i s i n t e r c a l a t e d b e t w e e n e a c h s i l i c a t e t e t r a h e d r a s h e e t, a s i l l u s t r a t e d i n F i g u r e 4. T h e s i l i c a t e a n d b r u c i t e l a y e r s s h a r e o x y g e n a t o m s w h i c h w o u l d n o r m a l l y b e s e p a r a t e d b y d i s t a n c e s o f n m i n t h e s i l i c a t e l a y e r a n d n m i n t h e b r u c i t e l a y e r ( 4 ). T h i s m i s m a t c h o f O - O d i s t a n c e s i n d u c e s a c u r v a t u r e o f t h e s h e e t s, t h e i d e a l r a d i u s o f w h i c h h a s b e e n c a l c u l a t e d a s 8. 8 n m ( 1 4 ). T h e c u r v a t u r e o f t h e s h e e t s p r o p a g a t e s a l o n g a p r e f e r r e d a x i s l e a d i n g t o t h e f o r m a t i o n o f t h e t u b u l a r s t r u c t u r e f o u n d i n c h r y s o t i l e. T h e c o n c e n t r i c s h e e t s f o r m i n g t h e f i b e r s h a v e a c u r v a t u r e r a d i u s from 2.5 to 3.0 nm for the internal layers up to approximately 25 nm for the external layers, yielding unit fibers (fibrils) with external d i a m e t e r s r a n g i n g b e t w e e n 2 0 a n d 5 0 n m ( 1 5 ). E l e c t r o n m i c r o s c o p y s t u d i e s ( 1 5 ) h a v e a l s o s h o w n t h a t, i n t h e u n i t c h r y s o t i l e f i b e r cross-section, the layers may appear in a concentric or spiral arrangement. T h e s t a c k i n g o f t h e t e t r a h e d r a l a n d o c t a h e d r a l s h e e t s i n t h e c h r y s o t i l e s t r u c t u r e h a s b e e n s h o w n t o y i e l d t h r e e t y p e s o f c h r y s o t i l e f i b e r s ( 3, 4 ) : clino-chrysotile: monoclinic stacking of the layers, x p a r a l l e l t o f i b e r a x i s, m o s t a b u n d a n t f o r m ortho-chrysotile: orthorhombic stacking of the layers, x p a r a l l e l t o f i b e r a x i s para-chrysotile: two layer structure, 180 rotation of two-layer structures, y p a r a l l e l t o f i b e r a x i s T h e e x t e n t o f s u b s t i t u t i o n o f m a g n e s i u m a n d s i l i c o n b y o t h e r c a t i o n s i n t h e c h r y s o t i l e s t r u c t u r e i s l i m i t e d b y t h e s t r u c t u r a l s t r a i n t h a t w o u l d r e s u l t f r o m r e p l a c e m e n t w i t h i o n s h a v i n g i n a p p r o p r i a t e r a d i i. I n t h e o c t a h e d r a l l a y e r ( b r u c i t e ), m a g n e s i u m c a n b e s u b s t i t u t e d b y s e v e r a l d i v a l e n t i o n s, F e +2, Mn +2, or Ni +2. In the tetrahedral layer, silicon may be replaced by or Al +3 or rarely Fe +3. M o s t o f t h e o t h e r e l e m e n t s t h a t a r e r a r e l y f o u n d i n v e i n f i b e r s a m p l e s, o r i n i n d u s t r i a l a s b e s t o s f i b e r s, a r e a s s o c i a t e d w i t h i n t e r s t i t i a l m i n e r a l p h a s e s. T y p i c a l c o m p o s i t i o n s o f b u l k c h r y s o t i l e f i b e r s f r o m d i f f e r e n t l o c a t i o n s a r e g i v e n i n T a b l e Amphiboles T h e c r y s t a l l i n e s t r u c t u r e c o m m o n t o a m p h i b o l e m i n e r a l s c o n s i s t s o f t w o r i b b o n s o f s i l i c a t e t e t r a h e d r a p l a c e d b a c k t o b a c k a s s h o w n 7

8 i n F i g u r e 5. T h e p l a n e o f a n i o n i c v a l e n c y s i t e s c r e a t e d b y t h i s d o u b l e r i b b o n a r r a n g e m e n t i s n e u t r a l i z e d b y t h e m e t a l c a t i o n s. T h e c r y s t a l s t r u c t u r e h a s s i x t e e n c a t i o n i c s i t e s o f f o u r d i f f e r e n t t y p e s ; t h e s e s i t e s c a n h o s t a l a r g e v a r i e t y o f m e t a l c a t i o n s w i t h o u t s u b s t a n t i a l d i s r u p t i o n o f t h e l a t t i c e ( s e e s e c t i o n 3. 2 a n d F i g. 5 ). I n c o n t r a s t t o c h r y s o t i l e f i b e r s, t h e a t o m i c c r y s t a l s t r u c t u r e o f a m p h i b o l e s d o e s n o t i n h e r e n t l y l e a d t o f i b e r f o r m a t i o n. T h e f o r m a t i o n o f a s b e s t i f o r m a m p h i b o l e s m u s t r e s u l t f r o m m u l t i p l e n u c l e a t i o n a n d s p e c i f i c g r o w t h c o n d i t i o n s. T h e d i f f e r e n c e b e t w e e n a s b e s t i f o r m a n d m a s s i v e a m p h i b o l e m i n e r a l s i s o b v i o u s o n t h e m a c r o s c o p i c s c a l e a l t h o u g h, t h e c r y s t a l l i n e s t r u c t u r e s o f t h e t w o v a r i e t i e s d o n o t e x h i b i t s u b s t a n t i a l d i f f e r e n c e s. T h e a s b e s t i f o r m a m p h i b o l e s t e n d t o h a v e a l a r g e r n u m b e r o f c r y s t a l d e f e c t s ( W a d s l e y d e f e c t s a n d t w i n n i n g a n d c h a i n w i d t h d i s o r d e r ) t h a n t h e n o n a s b e s t i f o r m v a r i e t i e s. T h e f r e q u e n c y a n d w i d t h o f t h e s e d e f e c t s v a r y w i t h a m p h i b o l e type (3, 10). Amphibole minerals, in general, are characterized by prismatic cleavage planes parallel to the c a x i s w h i c h i n t e r s e c t a t a n a n g l e o f a b o u t 5 6. H e n c e, i n t h e c r u s h i n g o f m a s s i v e, n o n f i b r o u s a m p h i b o l e s, m i c r o s c o p i c f r a g m e n t s a r e f o u n d h a v i n g t h e a p p e a r a n c e o f a s b e s t o s f i b e r s. H o w e v e r, t h e s t a t i s t i c a l a v e r a g e o f t h e i r a s p e c t r a t i o i s c o n s i d e r a b l y l o w e r t h a n t h a t o f t h e a s b e s t i f o r m a m p h i b o l e s. 5. PROPERTIES OF ASBESTOS FIBERS A s b e s t o s f i b e r s u s e d i n m o s t i n d u s t r i a l a p p l i c a t i o n s c o n s i s t o f a g g r e g a t e s o f s m a l l e r u n i t s ( f i b r i l s ). T h i s i s m o s t e v i d e n t i n c h r y s o t i l e t h a t e x h i b i t s a n i n h e r e n t, w e l l - d e f i n e d u n i t f i b e r. D i a m e t e r s o f f i b e r b u n d l e s i n b u l k i n d u s t r i a l s a m p l e s m a y b e i n t h e m i l l i m e t e r r a n g e i n s o m e c a s e s ; f i b e r b u n d l e l e n g t h s m a y b e s e v e r a l m i l l i m e t e r s t o 1 0 c m o r m o r e. T h e m e c h a n i c a l p r o c e s s e s e m p l o y e d t o e x t r a c t t h e f i b e r s f r o m t h e h o s t m a t r i x, o r t o f u r t h e r s e p a r a t e ( d e f i b e r i z e, o p e n ) t h e a g g r e g a t e s, c a n i m p a r t s i g n i f i c a n t m o r p h o l o g i c a l a l t e r a t i o n s t o t h e r e s u l t i n g f i b e r s. T y p i c a l l y, m i c r o s c o p i c o b s e r v a t i o n s o n m e c h a n i c a l l y o p e n e d f i b e r s r e v e a l f i b e r b e n d s a n d k i n k s, p a r t i a l s e p a r a t i o n o f a g g r e g a t e s, f i b e r e n d - s p l i t t i n g, e t c. T h e r e s u l t i n g p r o d u c t t h u s e x h i b i t s a w i d e v a r i e t y o f m o r p h o l o g i c a l f e a t u r e s, s o m e o f w h i c h c a n b e s e e n i n F i g u r e 2. M o r p h o l o g i c a l v a r i a n c e s o c c u r m o r e f r e q u e n t l y w i t h c h r y s o t i l e t h a n a m p h i b o l e s. T h e c r y s t a l s t r u c t u r e o f c h r y s o t i l e, i t s h i g h e r f l e x i b i l i t y, a n d i n t e r f i b r i l a d h e s i o n ( 3 ) a l l o w f o r a v a r i e t y o f i n t e r m e d i a t e s h a p e s w h e n f i b e r a g g r e g a t e s a r e s u b j e c t e d t o m e c h a n i c a l s h e a r. A m p h i b o l e f i b e r s a r e g e n e r a l l y m o r e b r i t t l e a n d a c c o m m o d a t e l e s s m o r p h o l o g i c a l d e f o r m a t i o n d u r i n g m e c h a n i c a l t r e a t m e n t F i b e r L e n g t h D i s t r i b u t i o n F o r i n d u s t r i a l a p p l i c a t i o n s, t h e f i b e r l e n g t h a n d l e n g t h d i s t r i b u t i o n a r e o f p r i m a r y i m p o r t a n c e b e c a u s e t h e y a r e c l o s e l y r e l a t e d t o t h e p e r f o r m a n c e o f t h e f i b e r s i n m a t r i x r e i n f o r c e m e n t. V a r i o u s f i b e r c l a s s i f i c a t i o n m e t h o d s h a v e t h u s b e e n d e v i s e d. R e p r e s e n t a t i v e d i s t r i b u t i o n s o f f i b e r l e n g t h s a n d d i a m e t e r s c a n b e o b t a i n e d t h r o u g h m e a s u r e m e n t a n d s t a t i s t i c a l a n a l y s i s o f m i c r o p h o t o g r a p h s ( 1 5 ) ; f i b e r l e n g t h d i s t r i b u t i o n s h a v e a l s o b e e n o b t a i n e d f r o m a u t o m a t e d o p t i c a l a n a l y z e r s ( 1 6 ). T y p i c a l f i b e r l e n g t h d i s t r i b u t i o n s o b t a i n e d f r o m t h e s e a p p r o a c h e s a r e i l l u s t r a t e d i n F i g u r e 6 f o r c h r y s o t i l e f i b e r s. A s i n t h e c a s e s s h o w n t h e r e, i n d u s t r i a l a s b e s t o s f i b e r s a m p l e s u s u a l l y c o n t a i n a r a t h e r b r o a d d i s t r i b u t i o n o f f i b e r l e n g t h s P h y s i c o - C h e m i c a l P r o p e r t i e s T h e i n d u s t r i a l a p p l i c a t i o n s o f c h r y s o t i l e f i b e r s t a k e a d v a n t a g e o f a c o m b i n a t i o n o f p r o p e r t i e s : f i b r o u s m o r p h o l o g y, h i g h t e n s i l e s t r e n g t h, r e s i s t a n c e t o h e a t a n d c o r r o s i o n, l o w e l e c t r i c a l c o n d u c t i v i t y, a n d h i g h f r i c t i o n c o e f f i c i e n t. I n m a n y a p p l i c a t i o n s, t h e s u r f a c e p r o p e r t i e s o f t h e f i b e r s a l s o p l a y a n i m p o r t a n t r o l e ; i n s u c h c a s e s, a d i s t i n c t i o n b e t w e e n c h r y s o t i l e a n d a m p h i b o l e s c a n b e o b s e r v e d b e c a u s e o f d i f f e r e n c e s i n t h e i r c h e m i c a l c o m p o s i t i o n a n d s u r f a c e m i c r o s t r u c t u r e. T e c h n o l o g i c a l l y r e l e v a n t p h y s i c a l a n d c h e m i c a l p r o p e r t i e s o f a s b e s t o s f i b e r s a r e g i v e n i n T a b l e 4. T h e r m a l B e h a v i o r A s b e s t o s f i b e r m i n e r a l s a r e h y d r a t e d s i l i c a t e s s o t h e i r b e h a v i o r a s a f u n c t i o n o f t e m p e r a t u r e i s r e l a t e d f i r s t t o d e h y d r a t i o n ( o r dehydroxylation) reactions. In the case of chrysotile, the crystalline structure is stable up to approximately 550 C (depending on the h e a t i n g p e r i o d ( 1 7 ) ), w h e r e t h e d e h y d r o x y l a t i o n o f t h e b r u c i t e l a y e r b e g i n s. T h i s p r o c e s s i s c o m p l e t e d n e a r C a n d i s c h a r a c t e r i z e d b y a t o t a l w e i g h t l o s s o f 1 3 %. T h e r e s u l t i n g m a g n e s i u m s i l i c a t e r e c r y s t a l l i z e s t o f o r m f o r s t e r i t e a n d s i l i c a i n t h e t e m p e r a t u r e r a n g e C, a s a n e x o t h e r m i c p r o c e s s. T h e w e i g h t l o s s a s a f u n c t i o n o f h e a t i n g t e m p e r a t u r e ( t h e r m o g r a v i m e t r y [ T G A ] ) a n d t h e r e a c t i o n h e a t s ( d i f f e r e n t i a l t h e r m a l a n a l y s i s [ D T A ] ) a r e i l l u s t r a t e d f o r c h r y s o t i l e i n F i g u r e 7 a. T h e s t r o n g l y e n d o t h e r m i c d e h y d r a t i o n p r o c e s s e n h a n c e s t h e h i g h t e m p e r a t u r e t h e r m a l i n s u l a t i o n p r o p e r t i e s o f c h r y s o t i l e a s b e s t o s. T h e b e h a v i o r o f a m p h i b o l e f i b e r s u n d e r c o n t i n u o u s h e a t i n g i s s i m i l a r t o t h a t o b s e r v e d w i t h c h r y s o t i l e, a l t h o u g h t h e t e m p e r a t u r e s o f dehydroxylation and recrystallization processes are different. The amphiboles have a lower water (hydroxyl) content and their d e h y d r o x y l a t i o n r e a c t i o n b e g i n s b e t w e e n C, d e p e n d i n g o n t h e a m p h i b o l e t y p e ; t h i s r e a c t i o n l e a d s t o a w e i g h t l o s s o f a p p r o x i m a t e l y 2 %. T h e l a t t e r i s i l l u s t r a t e d i n F i g u r e 7 b, t o g e t h e r w i t h a D T A r e c o r d i n g o f t h e a s s o c i a t e d t h e r m a l e v e n t s. T h e p r o d u c t s r e s u l t i n g f r o m t h e t h e r m a l d e c o m p o s i t i o n o f t h e a m p h i b o l e s a r e p y r o x e n e s, m a g n e t i t e, h e m a t i t e, a n d s i l i c a. I n t h e p r e s e n c e o f o x y g e n, t h e t h e r m a l d e c o m p o s i t i o n o f a m p h i b o l e s i s a s s o c i a t e d w i t h a n o x i d a t i o n o f d i v a l e n t i r o n t o t r i v a l e n t i r o n, w h i c h m a y l e a d t o a n i n c r e a s e i n t h e s a m p l e w e i g h t. T h e o x i d a t i o n p r o c e s s a l s o i n d u c e s a n o b v i o u s c o l o r a l t e r a t i o n : t h e f i b e r s a c q u i r e t h e c h a r a c t e r i s t i c f e r r i c o x i d e c o l o r. 8

9 A s b e s t o s f i b e r s, i n p a r t i c u l a r c h r y s o t i l e f i b e r s, c a n u n d e r g o s u b s t a n t i a l t h e r m a l d e g r a d a t i o n d u r i n g m e c h a n i c a l g r i n d i n g. I n h i g h - e n e r g y a t t r i t i o n e q u i p m e n t, s u c h a s b a l l m i l l s, t h e h i g h l o c a l i z e d i m p a c t e n e r g i e s c a n c a u s e t h e c r y s t a l l i n e s t r u c t u r e t o b e c o m e a m o r p h o u s. T h i s i s p a r t i c u l a r l y t r u e f o r d r y m i l l i n g o r m i l l i n g i n o r g a n i c s o l v e n t s. H o w e v e r, m i l l i n g i n a q u e o u s s l u r r i e s i s l e s s d e t r i m e n t a l, w a t e r o f f e r i n g s o m e p r o t e c t i o n a g a i n s t h i g h i m p a c t d e g r a d a t i o n ( 1 8 ). T e n s i l e S t r e n g t h T h e i n h e r e n t t e n s i l e s t r e n g t h o f a s i n g l e a s b e s t o s f i b e r, b a s e d o n t h e s t r e n g t h o f S i - O - S i b o n d s i n t h e s i l i c a t e c h a i n, s h o u l d b e n e a r 1 0 g i g a p a s c a l s ( G P a ) ( x p o u n d s p e r s q u a r e i n c h [ p s i ] ) ( 1 9 ). H o w e v e r, i n d u s t r i a l f i b e r s e x h i b i t s u b s t a n t i a l l y l o w e r v a l u e s, b e c a u s e o f t h e p r e s e n c e o f v a r i o u s t y p e s o f s t r u c t u r a l o r c h e m i c a l d e f e c t s. T h e m e a s u r e d t e n s i l e s t r e n g t h o f c h r y s o t i l e f i b e r s h a s b e e n r e p o r t e d i n t h e r a n g e G P a ( 1 6 0, , p s i ). T h e a c c u r a t e d e t e r m i n a t i o n o f t h i s p a r a m e t e r i s d i f f i c u l t s i n c e t h e m e a s u r e m e n t p e r f o r m e d o n a f i b e r a g g r e g a t e i s i n f l u e n c e d b y i n t e r f i b r i l a d h e s i o n, d i s c o n t i n u i t i e s i n s o m e o f t h e u n i t f i b e r s, m i n e r a l i n c l u s i o n s, e t c. C o n s e q u e n t l y, h i g h e r t e n s i l e s t r e n g t h r e s u l t s a r e o b t a i n e d f o r m e a s u r e m e n t s d o n e o n s h o r t a n d t h i n f i b e r s ( 2 0, 2 1 ). T h e t e n s i l e s t r e n g t h s o f a m o s i t e a n d c r o c i d o l i t e a r e c o m p a r a b l e t o t h a t o f c h r y s o t i l e. W i t h a m p h i b o l e s, t h e t e n s i l e s t r e n g t h i s h i g h l y i n f l u e n c e d b y t h e i r o n c o n t e n t s i n c e i r o n - o x y g e n b o n d s l o c a t e d i n t h e f i b e r a x e s, p a r t i c u l a r l y t h o s e i n v o l v i n g t r i v a l e n t F e, a r e p a r t i c u l a r l y s t r o n g. T h e t r e n d o b s e r v e d o f i n c r e a s i n g t e n s i l e s t r e n g t h o f a m p h i b o l e s f r o m t r e m o l i t e, t o a m o s i t e, t o c r o c i d o l i t e i s d i r e c t l y r e l a t e d t o t h e i r o n c o n t e n t o f t h e s e f i b e r s ( 2 2 ). T h e v a r i a t i o n i n t e n s i l e s t r e n g t h o f a s b e s t o s f i b e r s a s a f u n c t i o n o f t e m p e r a t u r e a l s o s h a r p l y d i s t i n g u i s h e s c h r y s o t i l e a n d a m p h i b o l e s. C h r y s o t i l e r e t a i n s ( a n d e v e n s l i g h t l y i n c r e a s e s ) i t s t e n s i l e s t r e n g t h u p t o C, u n t i l t h e d e h y d r o x y l a t i o n r e a c t i o n b e g i n s ; i t d r o p s s h a r p l y a t h i g h e r t e m p e r a t u r e s. A m p h i b o l e s, o n t h e o t h e r h a n d, e x h i b i t a d e c r e a s i n g t e n s i l e s t r e n g t h b e g i n n i n g a t a p p r o x i m a t e l y C. F o r e x a m p l e, a t C, c r o c i d o l i t e h a s l o s t 5 0 % o f i t s i n i t i a l t e n s i l e s t r e n g t h ( 2 2 ). A s b e s t o s F i b e r s i n A q u e o u s M e d i a A l t h o u g h a s b e s t o s f i b e r s c a n n o t b e v i e w e d a s w a t e r - s o l u b l e s i l i c a t e s, p r o l o n g e d e x p o s u r e o f c h r y s o t i l e o r a m p h i b o l e s t o w a t e r ( e s p e c i a l l y a t h i g h t e m p e r a t u r e ) l e a d s t o s l o w p r o g r e s s i v e l e a c h i n g o f b o t h t h e i r m e t a l a n d s i l i c a t e c o m p o n e n t s ( 2 3 ). I n t h e c a s e o f chrysotile fibers (in a given amount of water), the brucite layer will, fairly rapidly, dissolve in part, with concomitant increase in the ph o f t h e s o l u t i o n. T h e e q u i l i b r i u m p H v a l u e f o r a n a q u e o u s c h r y s o t i l e s l u r r y r e a c h e s F r e e b r u c i t e, p r e s e n t a s c o n t a m i n a n t i n t h e f i b e r s, a l s o c o n t r i b u t e s t o t h e p H i n c r e a s e. I n c o n t a c t w i t h s o l u t i o n s o f m i n e r a l a c i d s, o r g a n i c a c i d s, o r m a g n e s i u m c o m p l e x i n g a g e n t s, t h e r a t e o f d i s s o l u t i o n o f t h e b r u c i t e l a y e r s i s i n c r e a s e d. W h e n c a r r i e d o u t t o t h e e x t r e m e, t h e l e a c h i n g o f m a g n e s i u m l e a d s t o a w e i g h t l o s s o f 5 8 %. T h e r e s i d u a l s i l i c a i s l a r g e l y a m o r p h o u s a n d, a l t h o u g h t h e f i b e r s r e t a i n a n a p p a r e n t f i b r o u s m o r p h o l o g y, t h e i r m e c h a n i c a l r e s i s t a n c e i s d r a s t i c a l l y r e d u c e d. T h e d i s p e r s i o n o f a m p h i b o l e s i n c o n c e n t r a t e d H C l s o l u t i o n s a l s o l e a d s t o p a r t i a l l e a c h i n g, t h e r a t e o f w h i c h d e p e n d s o n t h e m e t a l c a t i o n s p r e s e n t. W i t h c r o c i d o l i t e, o n l y s m a l l a m o u n t s o f m a g n e s i u m a n d s o d i u m a r e e x t r a c t e d i n t h e s e c o n d i t i o n s, w h e r e a s a m o s i t e l i b e r a t e s s u b s t a n t i a l q u a n t i t i e s o f i r o n a n d m a g n e s i u m. O v e r a l l, t r e m o l i t e a p p e a r s t o e x h i b i t t h e h i g h e s t r e s i s t a n c e t o a c i d l e a c h i n g ( 3 ). O n t h e o t h e r h a n d, b o t h c h r y s o t i l e a n d t h e a m p h i b o l e s e x h i b i t a h i g h d e g r e e o f c h e m i c a l i n e r t i a t o w a r d s s t r o n g a l k a l i e s o v e r extended periods. At high temperatures, reactions with alkalies (NaOH, KOH, Ca(OH) 2 ) become significant over relatively short p e r i o d s ; f o r e x a m p l e, c r o c i d o l i t e w a s r e p o r t e d t o b e a t t a c k e d b y p o t a s s i u m h y d r o x i d e a b o v e C ( 2 3 ). O t h e r B u l k P h y s i c a l P r o p e r t i e s T h e h a r d n e s s o f a s b e s t o s f i b e r s i s c o m p a r a b l e t o t h a t o f o t h e r c r y s t a l l i n e o r g l a s s y s i l i c a t e s. C o m p a r e d t o g l a s s f i b e r s, a m p h i b o l e s h a v e s i m i l a r h a r d n e s s v a l u e s, w h i l e c h r y s o t i l e s h o w s l o w e r h a r d n e s s v a l u e s ( 2 2 ). T h e f r i c t i o n c o e f f i c i e n t s o f a s b e s t o s f i b e r s a r e a l s o d i f f e r e n t f o r c h r y s o t i l e a n d a m p h i b o l e s ( w h e n m e a s u r e d a g a i n s t t h e s a m e m a t e r i a l ). C o m p a r e d t o g l a s s f i b e r s, t h e f r i c t i o n c o e f f i c i e n t s d e c r e a s e i n t h e o r d e r : c h r y s o t i l e, a m p h i b o l e s, g l a s s f i b e r s. T h e h i g h e l e c t r i c a l r e s i s t i v i t y o f a s b e s t o s f i b e r s i s w e l l k n o w n a n d h a s b e e n w i d e l y e x p l o i t e d i n e l e c t r i c a l i n s u l a t i o n a p p l i c a t i o n s. I n g e n e r a l, t h e r e s i s t i v i t y o f c h r y s o t i l e i s l o w e r t h a n t h a t o f t h e a m p h i b o l e s, p a r t i c u l a r l y i n h i g h h u m i d i t y e n v i r o n m e n t s b e c a u s e o f t h e a v a i l a b i l i t y o f s o l u b l e i o n s. F o r e x a m p l e, t h e e l e c t r i c a l r e s i s t i v i t y o f c h r y s o t i l e d e c r e a s e s f r o m 1 t o m e g a o h m s p e r c m ( MS/ c m ) i n a d r y e n v i r o n m e n t t o v a l u e s o f t o 0. 4 MS/ c m a t 9 1 % r e l a t i v e h u m i d i t y. A m p h i b o l e s, o n t h e o t h e r h a n d, e x h i b i t r e s i s t i v i t y b e t w e e n 8, a n d 9 0 0, S/cm. With respect to magnetic properties, the intrinsic magnetic susceptibility of pure chrysotile is very weak. However, the presence of a s s o c i a t e d m i n e r a l s s u c h a s m a g n e t i t e, a s w e l l a s s u b s t i t u t i o n i o n s ( F e, M n ), i n c r e a s e s t h e m a g n e t i c s u s c e p t i b i l i t y t o v a l u e s a r o u n d 1. 9 t o 3. 5 x / g r a m o e r s t e d ( g O e ). W i t h a m p h i b o l e s, t h e m a g n e t i c s u s c e p t i b i l i t y i s m u c h h i g h e r, m a i n l y b e c a u s e o f t h e h i g h i r o n c o n t e n t ; t y p i c a l l y, a m o s i t e a n d c r o c i d o l i t e e x h i b i t s u s c e p t i b i l i t y v a l u e s o f 6 9 t o 7 1 x /g Oe (24). 9

10 5. 3 S u r f a c e P r o p e r t i e s Surface Area T h e s p e c i f i c s u r f a c e a r e a o f i n d u s t r i a l a s b e s t o s f i b e r s o b v i o u s l y d e p e n d s o n t h e e x t e n t o f t h e i r d e f i b e r i z a t i o n ( o p e n i n g ), a n d i s usually between 1 and 30 meters squared per gram (m 2 /g). As measured by BET (Brunauer, Emmett, and Teller) nitrogen adsorption, c h r y s o t i l e f i b e r s e x h i b i t s u r f a c e a r e a s b e t w e e n 1 5 a n d 3 0 m 2 / g. W i t h r e g a r d t o a m p h i b o l e f i b e r s, s u r f a c e a r e a s o f 1. 8 t o 9 m 2 /g have b e e n r e p o r t e d f o r c r o c i d o l i t e a n d 1. 3 t o 5. 5 m 2 / g f o r a m o s i t e ( 2 5 ). S u c h a d i f f e r e n c e o r i g i n a t e s, i n p a r t, i n t h e r e l a t i v e s i z e s o f t h e u n i t f i b e r s o f c h r y s o t i l e a n d t h e a m p h i b o l e s ; h o w e v e r, t h e r e s u l t s a l s o r e f l e c t t h e a b i l i t y o f n i t r o g e n m o l e c u l e s t o d i f f u s e b e t w e e n t h e u n i t f i b e r s o f a l a r g e r a g g r e g a t e. I n r e l a t i o n t o m a g n e s i u m a c i d l e a c h i n g o f c h r y s o t i l e d i s c u s s e d e a r l i e r, t h e s u r f a c e a r e a o f a n a m o r p h o u s silica resulting from extensive acid leaching may reach 450 m 2 /g (26). A l s o, t h e a d s o r p t i o n o f a n i o n i c o r n e u t r a l s u r f a c t a n t s o n c h r y s o t i l e f i b e r s i n a q u e o u s d i s p e r s i o n s e n h a n c e s f i b e r s e p a r a t i o n, w i t h a c o n c o m i t a n t i n c r e a s e o f s u r f a c e a r e a ( 2 7 ). S u c h e f f e c t s h a v e n o t b e e n r e p o r t e d f o r a m p h i b o l e f i b e r s. S u r f a c e C h a r g e i n A q u e o u s M e d i a B e c a u s e o f d i s s o l u t i o n - i o n i z a t i o n e f f e c t s, t h e s u r f a c e o f a s b e s t o s f i b e r s i n a q u e o u s d i s p e r s i o n s a d o p t s a n e l e c t r o s t a t i c c h a r g e. I n t h e c a s e o f c h r y s o t i l e, p a r t i a l d i s s o l u t i o n o f t h e b r u c i t e l a y e r l e a d s t o a p o s i t i v e s u r f a c e c h a r g e ( o r p o t e n t i a l ), w h i c h i s s t r o n g l y i n f l u e n c e d b y t h e s o l u t i o n p H. T h e i s o e l e c t r i c p o i n t o f c h r y s o t i l e ( p H o f z e r o s u r f a c e c h a r g e ) h a s b e e n r e p o r t e d a s I n t h e c a s e o f a m p h i b o l e f i b e r s, t h e s u r f a c e c h a r g e s e e m s d o m i n a t e d b y t h e s i l i c a c o m p o n e n t, a n d i s g e n e r a l l y o b s e r v e d t o b e n e g a t i v e, i n c r e a s i n g t o w a r d s 0 a s the ph is decreased. Since the progressive leaching of magnesium from the external brucite layer of chrysotile gradually exposes silica, t h e s u r f a c e p o t e n t i a l r a p i d l y d e c r e a s e s e a r l y i n t h e l e a c h i n g r e a c t i o n ( 3 ). A d s o r p t i o n a n d S u r f a c e C h e m i c a l G r a f t i n g A s w i t h s i l i c a a n d m a n y o t h e r s i l i c a t e m i n e r a l s, t h e s u r f a c e o f a s b e s t o s f i b e r s e x h i b i t s a s i g n i f i c a n t c h e m i c a l r e a c t i v i t y. I n p a r t i c u l a r, t h e h i g h l y p o l a r s u r f a c e o f c h r y s o t i l e f i b e r s p r o m o t e s a d s o r p t i o n ( p h y s i - o r c h e m i s o r p t i o n ) o f v a r i o u s t y p e s o f o r g a n i c o r i n o r g a n i c s u b s t a n c e s ( 2 3 ). M o r e o v e r, s p e c i f i c c h e m i c a l r e a c t i o n s c a n b e p e r f o r m e d w i t h t h e s u r f a c e f u n c t i o n a l g r o u p s ( O H g r o u p s f r o m b r u c i t e o r e x p o s e d s i l i c a ). T h e c h e m i c a l r e a c t i o n o f c o u p l i n g a g e n t s, s u c h a s o r g a n o s i l a n e c o m p o u n d s, o n c h r y s o t i l e y i e l d s f i b e r s w i t h a h i g h l y h y d r o p h o b i c s u r f a c e. T h r o u g h a n a d e q u a t e c h o i c e o f c o u p l i n g a g e n t s ( o r g a n o s i l a n e s o r o t h e r s ), t h e f i b e r m a t r i x i n t e r a c t i o n s i n c o m p o s i t e m a t e r i a l s c a n t h u s b e o p t i m i z e d ( 2 8 ). S u r f a c e c h e m i c a l m o d i f i c a t i o n s c a n b e c a r r i e d o u t i n f i b e r s l u r r i e s o r t h r o u g h g a s - s o l i d r e a c t i o n s ; f o r example the gas-phase reaction of POCl 3 w i t h c h r y s o t i l e, l e a d s t o a s u r f a c e c o v e r a g e w i t h i n s o l u b l e m a g n e s i u m p h o s p h a t e ( 2 9 ). T h e s u r f a c e p r o p e r t i e s a n d r e a c t i v i t y o f c h r y s o t i l e f i b e r s c a n a l s o b e m o d i f i e d b y c a t i o n i c s u b s t i t u t i o n, f o r e x a m p l e, r e p l a c i n g m a g n e s i u m b y a l u m i n u m ( 3 0 ) o r o t h e r m e t a l c a t i o n s ( 3 1 ). 6. ANALYTICAL METHODS AND IDENTIFICATION OF ASBESTOS FIBERS T h e i d e n t i f i c a t i o n o f a s b e s t o s f i b e r s c a n b e p e r f o r m e d t h r o u g h m o r p h o l o g i c a l e x a m i n a t i o n, t o g e t h e r w i t h s p e c i f i c a n a l y t i c a l m e t h o d s t o o b t a i n t h e m i n e r a l c o m p o s i t i o n a n d s t r u c t u r e. M o r p h o l o g i c a l c h a r a c t e r i z a t i o n i n i t s e l f u s u a l l y d o e s n o t c o n s t i t u t e a r e l i a b l e i d e n t i f i c a t i o n c r i t e r i o n ( 1 ). H e n c e, m i c r o s c o p i c e x a m i n a t i o n m e t h o d s a n d o t h e r a n a l y t i c a l a p p r o a c h e s a r e u s u a l l y c o m b i n e d M i c r o s c o p i c M e t h o d s T h e u s e o f m i c r o s c o p i c m e t h o d s i s p r e f e r r e d i n c a s e s w h e r e l i m i t e d q u a n t i t i e s o f s a m p l e a r e a v a i l a b l e, t y p i c a l l y i n a n a l y z i n g f i b e r s r e c o v e r e d f r o m s a m p l i n g o f a i r b o r n e d u s t. W i t h f i b e r s h a v i n g l e n g t h s i n e x c e s s o f 5 :m, o p t i c a l m i c r o s c o p y w i t h l i g h t p o l a r i z a t i o n ( p o l a r i z i n g l i g h t m i c r o s c o p y ) h a s p r o v e n a v e r y p o w e r f u l t e c h n i q u e. T h e o p t i c a l p r o p e r t i e s o f t h e d i f f e r e n t t y p e s o f a s b e s t o s f i b e r s, c o m b i n e d w i t h i n f o r m a t i o n o n f i b e r s h a p e s, e n a b l e p o s i t i v e i d e n t i f i c a t i o n o f a l l v a r i e t i e s o f a s b e s t o s f i b e r s. T h i s c a n b e c a r r i e d o u t e v e n w h e n t h e f i b e r s a r e m i x e d w i t h t h e i r n o n f i b r o u s a n a l o g u e o r w i t h v a r i o u s o t h e r m a t e r i a l s ( 3 2 ). O f t e n, r e f r a c t i v e i n d e x, c o l o r, p l e o c h r o i s m, b i r e f r i n g e n c e, o r i e n t a t i o n, e t c. c a n n o t b e m e a s u r e d a c c u r a t e l y, t h u s o t h e r m e t h o d s o f a n a l y s i s m u s t b e e m p l o y e d. A s b e s t o s f i b e r i d e n t i f i c a t i o n a l s o c a n b e a c h i e v e d t h r o u g h t r a n s m i s s i o n o r s c a n n i n g e l e c t r o n m i c r o s c o p y ( T E M, S E M ) t e c h n i q u e s t h a t a r e e s p e c i a l l y u s e f u l w i t h v e r y s h o r t f i b e r s, o r w i t h e x t r e m e l y s m a l l s a m p l e s. W i t h a p p r o p r i a t e p e r i p h e r a l i n s t r u m e n t a t i o n, t h e s e techniques can yield the elemental composition of the fibers using energy dispersive x-ray fluorescence, and the crystal structure from s e l e c t e d a r e a e l e c t r o n d i f f r a c t i o n ( S A E D ). B o t h c h e m i c a l c o m p o s i t i o n a n d c r y s t a l s t r u c t u r e s a r e r e q u i r e d f o r p o s i t i v e i d e n t i f i c a t i o n I n s t r u m e n t a l M e t h o d s f o r B u l k S a m p l e s W i t h b u l k f i b e r s a m p l e s, o r s a m p l e s o f m a t e r i a l s c o n t a i n i n g s i g n i f i c a n t a m o u n t s o f a s b e s t o s f i b e r s, a n u m b e r o f o t h e r i n s t r u m e n t a l a n a l y t i c a l m e t h o d s c a n b e u s e d t o h e l p i n t h e i d e n t i f i c a t i o n o f a s b e s t o s f i b e r s. T h e e l e m e n t a l c h a r a c t e r i z a t i o n o f m i n e r a l s c a n b e 1 0

11 accomplished using methods, such as x-ray fluorescence (XRF) and x-ray photo-electron spectroscopy (XPS). The x-ray diffraction t e c h n i q u e ( X R D ) e n a b l e s t h e a n a l y s t t o i d e n t i f y t h e c r y s t a l s t r u c t u r e o f t h e v a r i o u s t y p e s o f a s b e s t o s f i b e r s, a s w e l l a s t h e n a t u r e o f o t h e r m i n e r a l s a s s o c i a t e d w i t h t h e f i b e r s ( 1 2, ). T h e r m o a n a l y t i c a l m e t h o d s ( D T A, T G A ) o f t e n a i d i n t h e i d e n t i f i c a t i o n o f t h e t y p e o f a s b e s t o s f i b e r s ( F i g. 7 ). F o r e x a m p l e, t h e s t r o n g e x o t h e r m o b s e r v e d w i t h c h r y s o t i l e a t C c a n b e u s e d a s a r o u t i n e i n d i c a t o r f o r d e t e r m i n i n g t h e c h r y s o t i l e c o n t e n t o f t a l c ( 3, 8 ). T h e r m a l m e t h o d s a l s o a r e u s e f u l f o r d e t e r m i n i n g c e r t a i n m i n e r a l c o n t a m i n a n t s o f a s b e s t o s f i b e r s, f o r e x a m p l e b r u c i t e a n d c a l c i t e i n chrysotile. I n f r a r e d s p e c t r o s c o p y a l s o i s u s e d t o a n a l y z e s a m p l e s c o n t a i n i n g a s b e s t o s f i b e r s. A b s o r p t i o n b a n d s i n t h e i n f r a r e d s p e c t r u m a s s o c i a t e d w i t h a s b e s t o s f i b e r s a r e i n t h e c m -1 range (specific hydroxyl bands) and the and cm -1 ranges ( s p e c i f i c a b s o r p t i o n b a n d s f o r v a r i o u s s i l i c a t e m i n e r a l s ) ( 3 ). B e c a u s e o t h e r m i n e r a l s, i n c l u d i n g n o n a s b e s t i f o r m a m p h i b o l e s, a l s o a b s o r b w i t h i n t h e s e w a v e l e n g t h b a n d s, i n f r a r e d s p e c t r o s c o p y i s n o t a l w a y s d e f i n i t i v e f o r i d e n t i f y i n g a s b e s t o s. E a c h o f t h e s e m i c r o s c o p i c a n d b u l k m e t h o d s o f a n a l y s i s p r o v i d e c l u e s t o r e s o l v e t h e i d e n t i t y o f t h e a s b e s t o s t y p e. A s n o t e d e a r l i e r, h o w e v e r, b o t h c r y s t a l l o g r a p h i c a n d e l e m e n t a l d a t a a r e r e q u i r e d f o r a p o s i t i v e i d e n t i f i c a t i o n. 7. PRODUCTION A b r e a k o u t, b y m a j o r p r o d u c i n g c o u n t r i e s, o f t h e w o r l d a s b e s t o s p r o d u c t i o n i s s h o w n f r o m t o i n T a b l e 5 a n d t o t a l w o r l d p r o d u c t i o n f r o m t o i s s h o w n i n F i g u r e 8. D u r i n g t h e e a r l y s, t h e r e w a s a b r i e f p e r i o d o f s t a g n a t i o n i n w o r l d a s b e s t o s p r o d u c t i o n, m u c h o f w h i c h c a n b e a t t r i b u t e d t o r e d u c e d c o n s u m p t i o n a s s o c i a t e d w i t h t h e e c o n o m i c d e p r e s s i o n i n t h e U n i t e d S t a t e s. A t that time, Canada was the major producing country and was highly dependent on U.S. markets for sales. By 1935, production began to i n c r e a s e s l o w l y u n t i l t h e o n s e t o f W o r l d W a r I I. C a n a d a a c c o u n t e d f o r t h e b u l k o f t h e i n c r e a s e d p r o d u c t i o n t o n n a g e. D u r i n g W o r l d W a r I I, p r o d u c t i o n d e c l i n e d i n m o s t r e g i o n s o f t h e w o r l d e x c e p t C a n a d a a n d s o u t h e r n A f r i c a. U. S. d e m a n d s a b s o r b e d m u c h o f t h e i n c r e a s e d p r o d u c t i o n f r o m t h e s e r e g i o n s d u r i n g t h e w a r. F o l l o w i n g W o r l d W a r I I, w o r l d p r o d u c t i o n i n c r e a s e d a g a i n, f u e l e d b y e x p a n d i n g e c o n o m i e s, f i r s t i n t h e U n i t e d S t a t e s a n d e l s e w h e r e l a t e r. H o w e v e r, g r o w i n g o p p o s i t i o n t o t h e u s e o f a s b e s t o s, w h i c h b e g a n i n e a r n e s t i n t h e e a r l y s, s o o n b r o u g h t t h i s g r o w t h t o a h a l t. A f t e r , t h e r e w a s a d o w n t u r n i n w o r l d p r o d u c t i o n a n d c o n s u m p t i o n. I n t h e U n i t e d S t a t e s, a s b e s t o s r e g u l a t i o n s b e c a m e i n c r e a s i n g l y s t r i c t w i t h s i z a b l e r e d u c t i o n s i n p e r m i s s i b l e e x p o s u r e l i m i t s b e i n g e n a c t e d. L i a b i l i t y a l s o b e c a m e a m a j o r i s s u e. C o m p a n i e s t h a t m i n e d a s b e s t o s a n d t h o s e t h a t m a n u f a c t u r e d a s b e s t o s p r o d u c t s f a c e d a n i n c r e a s i n g n u m b e r o f c l a s s - a c t i o n l a w s u i t s a n d h a d d i f f i c u l t i e s o b t a i n i n g l i a b i l i t y i n s u r a n c e. C o m p o u n d i n g t h e p r o b l e m w a s t h e s l o w s h i f t t o a s b e s t o s s u b s t i t u t e s o r a l t e r n a t i v e p r o d u c t s ( i. e. d u c t i l e i r o n o r p o l y v i n y l c h l o r i d e ( P V C ) p i p e i n s t e a d o f a s b e s t o s - c e m e n t p i p e ) i n r e s p o n s e t o p u b l i c d e m a n d. A s a r e s u l t, U. S. c o n s u m p t i o n p e a k e d i n a t a b o u t 8 0 1, t o n s. W o r l d p r o d u c t i o n a n d c o n s u m p t i o n p e a k e d a t a b o u t 4. 8 x t o n s i n I n , U. S. c o n s u m p t i o n w a s 1 4, t o n s a n d w o r l d p r o d u c t i o n was 1.9 x 10 6 t o n s. S h i f t s i n p r o d u c t i o n b y t h e m a i n p r o d u c i n g c o u n t r i e s o c c u r r e d d u r i n g t h i s t i m e. C a n a d a w a s t h e d o m i n a n t p r o d u c e r d u r i n g t h e f i r s t h a l f o f t h e c e n t u r y. B y , t h e f o r m e r S o v i e t U n i o n h a d b e c o m e, a n d s t i l l r e m a i n s, t h e l a r g e s t p r o d u c i n g r e g i o n. B r a z i l, C h i n a, S o u t h A f r i c a, a n d Z i m b a b w e a l s o r o s e f r o m r e l a t i v e l y o b s c u r i t y t o b e c o m e m a j o r a s b e s t o s p r o d u c e r s. C u r r e n t p r o d u c t i o n h a s d e c l i n e d i n a l l m a j o r p r o d u c i n g c o u n t r i e s e x c e p t C h i n a d u e t o t h e o p p o s i t i o n t o t h e u s e o f a s b e s t o s. B r a z i l, C a n a d a, C h i n a, t h e f o r m e r S o v i e t U n i o n r e p u b l i c s o f R u s s i a a n d K a z a k h s t a n, a n d Z i m b a b w e n o w a c c o u n t f o r m o r e t h a n 9 0 % o f t h e w o r l d p r o d u c t i o n ( 9 ). M o s t o f C h i n a ' s p r o d u c t i o n, a s w e l l a s t h e l i m i t e d p r o d u c t i o n o f m a n y o t h e r c o u n t r i e s, i s u s e d i n l o c a l i n d u s t r i a l a p p l i c a t i o n s. E s s e n t i a l l y a l l p r o d u c t i o n i s n o w c h r y s o t i l e. P r o d u c t i o n o f a m o s i t e a n d c r o c i d o l i t e c e a s e d i n t h e m i d s. S m a l l a m o u n t s o f a c t i n o l i t e a s b e s t o s, a n t h o p h y l l i t e a s b e s t o s, a n d t r e m o l i t e a s b e s t o s p r o b a b l y a r e p r o d u c e d f o r l o c a l u s e i n a f e w c o u n t r i e s s u c h a s I n d i a, P a k i s t a n, a n d Turkey. 8. MINING AND MILLING TECHNOLOGIES T h e f i n d i n g a n d m a p p i n g o f c h r y s o t i l e a s b e s t o s o r e d e p o s i t s u s u a l l y r e l i e s o n m a g n e t o m e t r i c s u r v e y s l a r g e l y b e c a u s e m a g n e t i t e i s a s s o c i a t e d w i t h a s b e s t o s d e p o s i t s, e x c e p t i n t h e c a s e o f o r e b o d i e s l o c a t e d i n s e d i m e n t a r y f o r m a t i o n s. A s i n o t h e r m i n i n g o p e r a t i o n s, c o r e d r i l l i n g i s u s e d f o r a p r e c i s e e v a l u a t i o n o f t h e g r a d e a n d v o l u m e o f t h e o r e b o d y ( 3 5 ). T h e c h o i c e o f a p a r t i c u l a r m i n i n g m e t h o d d e p e n d s o n a n u m b e r o f p a r a m e t e r s, t y p i c a l l y t h e p h y s i c a l p r o p e r t i e s o f t h e h o s t m a t r i x, t h e f i b e r c o n t e n t o f t h e o r e, t h e a m o u n t o f s t e r i l e m a t e r i a l s, t h e p r e s e n c e o f c o n t a m i n a n t s, a n d t h e e x t e n t o f p o t e n t i a l f i b e r d e g r a d a t i o n d u r i n g t h e v a r i o u s m i n i n g o p e r a t i o n s ( 3 6 ). M o s t o f t h e a s b e s t o s m i n i n g o p e r a t i o n s a r e o f t h e o p e n p i t t y p e, u s i n g b e n c h d r i l l i n g t e c h n i q u e s. T h e f i b e r e x t r a c t i o n ( m i l l i n g ) p r o c e s s m u s t b e c h o s e n s o a s t o o p t i m i z e r e c o v e r y o f t h e f i b e r s i n t h e o r e, w h i l e m i n i m i z i n g r e d u c t i o n o f f i b e r l e n g t h. S i n c e t h e a s b e s t o s f i b e r s h a v e a c h e m i c a l c o m p o s i t i o n s i m i l a r t o t h a t o f t h e h o s t r o c k, t h e s e p a r a t i o n p r o c e s s e s m u s t r e l y o n d i f f e r e n c e s i n t h e p h y s i c a l p r o p e r t i e s b e t w e e n t h e f i b e r s a n d t h e h o s t r o c k r a t h e r t h a n o n d i f f e r e n c e s i n t h e i r c h e m i c a l p r o p e r t i e s (36). I n d r y m i l l i n g o p e r a t i o n s, w h i c h a r e c u r r e n t l y t h e m o s t w i d e l y u s e d, t h e o r e i s f i r s t c r u s h e d t o a n o m i n a l s i z e a n d t h e n d r i e d ( F i g. 9 ). F i b e r e x t r a c t i o n t h e n b e g i n s t h r o u g h a s e r i e s o f c r u s h i n g o p e r a t i o n s, e a c h f o l l o w e d b y a v a c u u m a s p i r a t i o n o f t h e o r e r u n n i n g o n a v i b r a t i n g s c r e e n. O n t h e l a t t e r, t h e f i b e r s r e l e a s e d f r o m t h e o r e h a v e a t e n d e n c y t o m o v e t o t h e s u r f a c e a n d, b e c a u s e o f t h e i r 1 1

12 aerodynamic properties, they can be readily collected into a vacuum system. The fibers recovered from consecutive vibrating screens a r e b r o u g h t t o c y c l o n e s e p a r a t o r s, a n d t h e a i r i s f i l t e r e d t o r e m o v e t h e f i n e r s u s p e n d e d f i b e r s. G e n e r a l l y, t h e c o n s e c u t i v e c r u s h i n g - a s p i r a t i o n s t e p s l i b e r a t e f i b e r s t h a t i n c r e a s i n g l y s h o r t e r. L o n g e r f i b e r s i n w i d e v e i n s a r e e a s i e r t o r e l e a s e a n d t h u s a r e r e c o v e r e d i n t h e e a r l y p h a s e s o f t h e m i l l i n g o p e r a t i o n. I n t h i s w a y, a p r i m a r y c l a s s i f i c a t i o n, o r g r a d i n g o f t h e fibers can be achieved. Various secondary processing systems may be used to further separate the fiber aggregates and to remove n o n f i b r o u s m i n e r a l d u s t. A l l f i b e r e x t r a c t i o n a n d c l a s s i f i c a t i o n o p e r a t i o n s a r e u s u a l l y c a r r i e d o u t u n d e r n e g a t i v e p r e s s u r e t o m i n i m i z e a i r b o r n e d u s t i n t h e w o r k i n g e n v i r o n m e n t. W e t m i l l i n g o p e r a t i o n s, w h e r e t h e a s b e s t o s i s d i s p e r s e d i n w a t e r a n d n o t d r i e d u n t i l a f t e r t h e f i n a l s e p a r a t i o n p r o c e s s i s c o m p l e t e d, o f f e r a d v a n t a g e s i n d u s t c o n t r o l a n d t h e s e p a r a t i o n o f m i n e r a l c o n t a m i n a n t s f r o m t h e f i b e r p r o d u c t. H o w e v e r, w e t p r o c e s s t e c h n o l o g y c u r r e n t l y i s u s e d i n o n l y a f e w s m a l l - s c a l e m i l l i n g o p e r a t i o n s. W i t h t h e d e c l i n e o f t h e a s b e s t o s m a r k e t s i n c e t h e s, t h e e c o n o m i c s of constructing new wet processing facilities, or even dry milling facilities, was not favorable. For the most part, any new capacity that c a m e o n l i n e w o r l d w i d e, w i t h t h e p o s s i b l e e x c e p t i o n o f C h i n a, i n t h e p a s t 2 0 y e a r s h a s i n v o l v e d t h e r e o p e n i n g o r e x p a n s i o n o f o l d m i l l i n g o p e r a t i o n s. 9. FIBER CLASSIFICATION AND STANDARD TESTING METHODS I n t h e b e n e f i c i a t i o n o f a s b e s t o s f i b e r s, s e v e r a l p a r a m e t e r s a r e c o n s i d e r e d c r i t i c a l l y i m p o r t a n t a n d a r e u s e d a s s t a n d a r d e v a l u a t i o n c r i t e r i a ( 3 7 ) : l e n g t h ( o r l e n g t h d i s t r i b u t i o n ), d e g r e e o f o p e n i n g a n d s u r f a c e a r e a, p e r f o r m a n c e i n c e m e n t r e i n f o r c e m e n t, a n d d u s t a n d g r a n u l e c o n t e n t. T h e m e a s u r e m e n t o f f i b e r l e n g t h i s i m p o r t a n t s i n c e t h e l e n g t h d e t e r m i n e s t h e p r o d u c t c a t e g o r y i n w h i c h t h e f i b e r s w i l l b e u s e d a n d, t o a l a r g e e x t e n t, t h e i r c o m m e r c i a l v a l u e D r y C l a s s i f i c a t i o n M e t h o d T h e m o s t w i d e l y a c c e p t e d m e t h o d f o r c h r y s o t i l e f i b e r l e n g t h c h a r a c t e r i z a t i o n i n t h e i n d u s t r y i s t h e Q u e b e c S t a n d a r d t e s t ( Q S ). T h i s i s a d r y s i e v i n g m e t h o d ( o n v i b r a t i n g s c r e e n s ) w h i c h e n a b l e s t h e f r a c t i o n a t i o n o f a n a s b e s t o s f i b e r s a m p l e i n t o f o u r f r a c t i o n s o f d e c r e a s i n g s i z e s ( g r e a t e r t h a n 2 m e s h, l e s s t h a n 2 a n d g r e a t e r t h a n o r e q u a l t o 4 m e s h, l e s s t h a n 4 a n d g r e a t e r t h a n o r e q u a l t o 1 0 m e s h, a n d l e s s t h a n 1 0 m e s h, w i t h m e s h d e f i n e d a s t h e n u m b e r o f o p e n i n g s p e r l i n e a r i n c h ). T h i s s t a n d a r d t e s t h a s b e e n u s e d a s a b a s i s f o r d e v e l o p i n g a g r a d e s y s t e m c o m p r i s i n g n i n e m a i n f i b e r g r a d e s. T h e g r a d i n g s y s t e m i d e n t i f i e s t h e l o n g e s t f i b e r s a s g r a d e 1 a n d t h e s h o r t e s t a s g r a d e 9. G r a d e s 1 a n d 2 d e s i g n a t e c r u d e a s b e s t o s f i b e r s t h a t a r e h a n d p i c k e d a n d e s s e n t i a l l y c o n t a i n l a r g e c l u s t e r s o r a g g r e g a t e s o f f i b e r s. G r a d e 1 f i b e r s a r e t h o s e w i t h l e n g t h s g r e a t e r t h a n o r e q u a l t o 3 / 4 i n. ( 1. 9 c m ). G r a d e 2 f i b e r s a r e t h o s e w i t h l e n g t h s l e s s t h a n 3 / 4 i n. a n d g r e a t e r t h a n o r e q u a l t o 3 / 8 i n. ( c m ). F i b e r g r a d e s b e t w e e n 3 a n d 9 ( d e c r e a s i n g l e n g t h s ) a r e k n o w n a s m i l l e d a s b e s t o s f i b e r s a n d r e p r e s e n t m o s t o f t h e p r o d u c t i o n. T h e f i b e r g r a d e s 3 t h r o u g h 7 a r e d e t e r m i n e d u s i n g t h e Q S t e s t, w h e r e a s g r a d e s 8 a n d 9, c o n t a i n i n g v e r y s h o r t f i b e r s, a r e u s u a l l y d e t e r m i n e d a c c o r d i n g t o t h e i r b u l k d e n s i t i e s. O t h e r f i b e r c l a s s i f i c a t i o n s c h e m e s h a v e b e e n d e v i s e d f o r c h r y s o t i l e f i b e r s, b u t h i s t o r i c a l l y t h e Q S g r a d e s y s t e m h a s b e e n u s e d a s a r e f e r e n c e ; o t h e r c l a s s i f i c a t i o n s c h e m e s u s u a l l y h a v e c o r r e s p o n d e n c e s c a l e s f o r c o n v e r s i o n t o t h e Q S v a l u e s. A m o s i t e c a n b e c l a s s i f i e d a c c o r d i n g t o t h e Q S g r a d e s y s t e m, b u t c r o c i d o l i t e r e q u i r e s a d i f f e r e n t s c h e m e m a i n l y d u e t o t h e h a r s h n e s s o f t h e s e f i b e r s W e t C l a s s i f i c a t i o n M e t h o d A s e c o n d i n d u s t r i a l l y i m p o r t a n t f i b e r l e n g t h e v a l u a t i o n t e c h n i q u e i s t h e B a u e r - M c N e t t ( B M N ) c l a s s i f i c a t i o n. I n t h i s m e t h o d, a f i b e r s l u r r y i s c i r c u l a t e d t h r o u g h a s e r i e s o f f o u r g r i d s w i t h d e c r e a s i n g o p e n i n g s i z e ( p o s i t i o n e d v e r t i c a l l y ), t h u s y i e l d i n g f i v e f r a c t i o n s ( + 4, + 1 4, + 3 5, , a n d m e s h ). A s i m i l a r m e t h o d, u s i n g s m a l l e r s a m p l e s a n d h o r i z o n t a l g r i d s h a s a l s o b e e n d e v e l o p e d a n d r e f e r r e d t o a s t h e T u r n e r - N e w a l l c l a s s i f i c a t i o n. C u r r e n t l y, t h e B M N c l a s s i f i c a t i o n a n d t h e Q S t e s t a r e t h e m o s t w i d e l y u s e d f i b e r c l a s s i f i c a t i o n t e c h n i q u e s. W h e r e a s t h e r e a r e q u a l i t a t i v e r e l a t i o n s h i p s b e t w e e n Q S a n d B M N, t h e r e i s n o q u a n t i t a t i v e c o r r e s p o n d e n c e. I t i s r e a d i l y u n d e r s t o o d t h a t t h e s e s t a n d a r d t e s t s d o n o t p r o v i d e a c c u r a t e d e f i n i t i o n o f t h e f i b e r l e n g t h s ; t h e c l a s s i f i c a t i o n a l s o r e f l e c t s t h e h y d r o d y n a m i c b e h a v i o r ( v o l u m e ) o f t h e f i b e r s, w h i c h, b e c a u s e o f t h e i r c o m p l e x s h a p e s, i s n o t r e a d i l y p r e d i c t a b l e. O t h e r c l a s s i f i c a t i o n t e c h n i q u e s t h a t p r o v i d e s o m e i n s i g h t o n f i b e r l e n g t h s a r e t h e R o - T a p t e s t, t h e S u t e r - W e b b C o m b, a n d t h e W a s h t e s t O t h e r F i b e r E v a l u a t i o n M e t h o d s T h e e x t e n t o f f i b e r s e p a r a t i o n ( f i b e r o p e n n e s s ) i s a n i m p o r t a n t e v a l u a t i o n c r i t e r i o n t h a t i s c o m m o n l y m e a s u r e d b y s e v e r a l techniques, namely air permeability, adsorbent gas volume, bulk density, and resilience (compression and recovery). The adsorption a n d r e t e n t i o n o f k e r o s e n e i s a l s o u s e d a s a m e a s u r e o f f i b e r o p e n n e s s a n d f i b e r a d s o r p t i o n c a p a c i t y ( 3 7 ). T h e f i l t r a t i o n b e h a v i o r o f a s b e s t o s f i b e r s i s e v a l u a t e d t h r o u g h a d r a i n a g e t e s t u s i n g, g e n e r a l l y, a s a t u r a t e d c a l c i u m s u l f a t e s o l u t i o n. T h i s t e s t i s d e s i g n a t e d a s f r e e n e s s a n d r e f l e c t s t h e w a t e r d r a i n a g e r a t e r e l a t e d t o a s b e s t o s - c e m e n t f a b r i c a t i o n p r o c e s s e s ( 3 7 ). T h e r e i n f o r c i n g c a p a c i t y o f a s b e s t o s f i b e r s i n a c e m e n t m a t r i x c o n s t i t u t e s a n o t h e r k e y c r i t e r i o n f o r t h e e v a l u a t i o n o f a s b e s t o s f i b e r s. P r e p a r i n g s a m p l e s o f a s b e s t o s - c e m e n t c o m p o s i t e s t h a t, a f t e r a s t a n d a r d c u r i n g p e r i o d, a r e t e s t e d f o r f l e x u r a l r e s i s t a n c e a s s e s s e s t h i s 1 2

13 p r o p e r t y. T h e m e a s u r e d r u p t u r e m o d u l i a r e c o n v e r t e d i n t o a p a r a m e t e r r e f e r r e d t o a s t h e f i b e r s t r e n g t h u n i t ( 3 7 ). F i n a l l y, o t h e r p r o p e r t i e s o f a s b e s t o s f i b e r s m a y b e e v a l u a t e d d e p e n d i n g o n t h e p o t e n t i a l a p p l i c a t i o n. T y p i c a l l y, t h e g r i t s a n d s p i c u l e c o n t e n t, t h e m a g n e t i c s u s c e p t i b i l i t y ( m a g n e t i c r a t i n g ), t h e c o n t e n t i n s o l u b l e c h l o r i d e s, a n d t h e h u m i d i t y l e v e l m a y b e o f p a r t i c u l a r i n t e r e s t i n s p e c i f i c a p p l i c a t i o n s. 10. INDUSTRIAL APPLICATIONS A s b e s t o s f i b e r s h a v e b e e n u s e d i n a b r o a d v a r i e t y o f i n d u s t r i a l a p p l i c a t i o n s. I n t h e p e a k p e r i o d o f a s b e s t o s c o n s u m p t i o n i n i n d u s t r i a l i z e d c o u n t r i e s, s o m e a p p l i c a t i o n s, o r t y p e s o f p r o d u c t s, h a v e b e e n l i s t e d. B e c a u s e o f r e c e n t r e s t r i c t i o n s a n d c h a n g e s i n e n d u s e - m a r k e t s, m o s t o f t h e s e a p p l i c a t i o n s h a v e b e e n a b a n d o n e d a n d t h e r e m a i n d e r i s p u r s u e d u n d e r s t r i c t l y r e g u l a t e d c o n d i t i o n s. T h e m a i n p r o p e r t i e s o f a s b e s t o s f i b e r s t h a t c a n b e e x p l o i t e d i n i n d u s t r i a l a p p l i c a t i o n s ( 1 1 ) a r e t h e i r t h e r m a l, e l e c t r i c a l, a n d s o u n d i n s u l a t i o n ; i n f l a m m a b i l i t y ; m a t r i x r e i n f o r c e m e n t ( c e m e n t, p l a s t i c, a n d r e s i n s ) ; a d s o r p t i o n c a p a c i t y ( f i l t r a t i o n, l i q u i d s t e r i l i z a t i o n ) ; w e a r a n d f r i c t i o n p r o p e r t i e s ( f r i c t i o n m a t e r i a l s ) ; a n d c h e m i c a l i n e r t i a ( e x c e p t i n a c i d s ). T h e s e p r o p e r t i e s h a v e l e d t o s e v e r a l m a i n c l a s s e s o f i n d u s t r i a l p r o d u c t s o r a p p l i c a t i o n s ( 8 ). L o o s e a s b e s t o s f i b e r s, o r f o r m u l a t i o n s c o n t a i n i n g a s b e s t o s f i b e r s f o r s p r a y c o a t i n g s, w e r e w i d e l y u s e d i n t h e b u i l d i n g i n d u s t r y f o r f i r e p r o t e c t i o n a n d h e a t o r s o u n d i n s u l a t i o n d u r i n g a n d f o l l o w i n g W o r l d W a r I I. S u c h a p p l i c a t i o n s u s e d m a i n l y c h r y s o t i l e o r a m o s i t e, a l t h o u g h c r o c i d o l i t e w a s c o m m o n l y u s e d i n E u r o p e, t h i s p r a c t i c e w a s d i s c o n t i n u e d i n t h e s b e c a u s e o f h e a l t h c o n c e r n s. A s b e s t o s f i b e r s a l s o h a v e b e e n w i d e l y u s e d f o r t h e f a b r i c a t i o n o f p a p e r s a n d f e l t s f o r f l o o r i n g a n d r o o f i n g p r o d u c t s, p i p e l i n e w r a p p i n g, e l e c t r i c a l i n s u l a t i o n, e t c. A s b e s t o s t e x t i l e s, c o m p r i s i n g y a r n, t h r e a d, c l o t h, t a p e, o r r o p e, a l s o f o u n d w i d e a p p l i c a t i o n i n t h e r m a l a n d e l e c t r i c a l i n s u l a t i o n a n d f r i c t i o n p r o d u c t s i n b r a k e o r c l u t c h p a d s. I n r e c e n t y e a r s, u s e o f a s b e s t o s i n t h e s e a p p l i c a t i o n s h a s d e c r e a s e d s i g n i f i c a n t l y. P r o d u c t i o n o f a s b e s t o s r o o f i n g a n d f l o o r i n g f e l t s h a s b e e n d i s c o n t i n u e d. T h e r e i n f o r c i n g p r o p e r t i e s o f a s b e s t o s f i b e r s h a v e b e e n w i d e l y e x p l o i t e d i n a s b e s t o s - c e m e n t p r o d u c t s m o s t l y b y t h e c o n s t r u c t i o n a n d w a t e r i n d u s t r i e s. A s b e s t o s - c e m e n t p r o d u c t s s u c h a s b o a r d, p i p e, a n d s h e e t r e p r e s e n t b y f a r t h e l a r g e s t w o r l d w i d e i n d u s t r i a l c o n s u m p t i o n o f a s b e s t o s f i b e r s, a n e s t i m a t e d 8 0 % o f t h e m a r k e t i n ( 3 8 ). W i t h m a r k e t c h a n g e s s i n c e , a s b e s t o s - c e m e n t p r o d u c t s n o w p r o b a b l y a c c o u n t f o r m o r e t h a n 9 8 % o f f i b e r s a l e s. A s b e s t o s f i b e r s a l s o h a v e b e e n u s e d t o r e i n f o r c e p l a s t i c p r o d u c t s m a d e f r o m P V C, p h e n o l i c s, p o l y p r o p y l e n e, n y l o n, e t c. R e i n f o r c e m e n t o f t h e r m o s e t a n d t h e r m o p l a s t i c r e s i n s b y a s b e s t o s f i b e r s w a s u s e d t o d e v e l o p p r o d u c t s f o r t h e a u t o m o t i v e, e l e c t r o n i c, a n d p r i n t i n g i n d u s t r i e s. E x c e p t f o r s o m e s p e c i a l t y p r o d u c t s, t h e u s e o f a s b e s t o s i n p l a s t i c s h a s e s s e n t i a l l y c e a s e d. T h e c o m b i n a t i o n o f a s b e s t o s f i b e r s w i t h v a r i o u s t y p e s o f n a t u r a l o r s y n t h e t i c r e s i n s h a s l e d t o t h e d e v e l o p m e n t o f a v a r i e t y o f p r o d u c t s a n d a p p l i c a t i o n s. A m o n g t h o s e, t h e i n c o r p o r a t i o n o f a s b e s t o s f i b e r s ( m a i n l y c h r y s o t i l e ) i n t o r u b b e r m a t r i c e s y i e l d s m a t e r i a l s t h a t w e r e w i d e l y u s e d f o r f a b r i c a t i o n o f p a c k i n g s a n d g a s k e t s. C o m p l e x f o r m u l a t i o n s, c o m p r i s i n g s h o r t a s b e s t o s f i b e r s ( u s u a l l y c h r y s o t i l e ), r e s i n s, a n d o t h e r f i l l e r s a n d m o d i f i e r s, h a v e b e e n d e v e l o p e d a s f r i c t i o n m a t e r i a l s f o r b r a k e l i n i n g s a n d p a d s. A s b e s t o s f i b e r s a l s o h a v e f o u n d b r o a d a p p l i c a t i o n a s r e i n f o r c i n g a g e n t s i n c o a t i n g s, s e a l a n t s, a n d a d h e s i v e f o r m u l a t i o n s. O f t h e s e a p p l i c a t i o n s, b r a k e l i n i n g s a n d p a d s, g a s k e t s, a n d r o o f c o a t i n g s c o m p r i s e t h e b u l k o f t h e c o n s u m p t i o n. F i n a l l y, t h e c o m b i n e d r e i n f o r c i n g e f f e c t a n d h i g h a b s o r p t i o n c a p a c i t y o f a s b e s t o s f i b e r s w e r e e x p l o i t e d i n a v a r i e t y o f a p p l i c a t i o n s t o i n c r e a s e d i m e n s i o n a l s t a b i l i t y, t y p i c a l l y i n v i n y l o r a s p h a l t t i l e s a n d a s p h a l t r o a d s u r f a c i n g. A s b e s t o s i s n o l o n g e r u s e d i n t h e s e a p p l i c a t i o n s. T a b l e 6 s u m m a r i z e s t h e v a r i o u s c l a s s e s o f a p p l i c a t i o n f o r a s b e s t o s f i b e r s i n c o m b i n a t i o n w i t h o t h e r m a t e r i a l s. T h e d i a g r a m s h o w s t h a t i n r e c e n t y e a r s, m o s t i n d u s t r i a l a p p l i c a t i o n s i n v o l v e a s b e s t o s f i b e r s b o n d e d w i t h i n a n o r g a n i c o r i n o r g a n i c m a t r i x. A s b e s t o s - c e m e n t p r o d u c t s a c c o u n t f o r t h e b u l k o f t h e w o r l d s a s b e s t o s u s a g e. I n t h e U n i t e d S t a t e s, t h e m a j o r u s e i s i n r o o f i n g c o m p o u n d s ( 6 2 % ), f o l l o w e d b y g a s k e t s ( 2 2 % ), a n d f r i c t i o n p r o d u c t s ( 1 1 % ). S m a l l a m o u n t s o f a s b e s t o s a l s o a r e u s e d t o m a n u f a c t u r e s o m e i n s u l a t i o n p r o d u c t s a n d w o v e n a n d p l a s t i c p r o d u c t s ( 9 ). E a c h t y p e o r g r o u p o f p r o d u c t s u s u a l l y r e q u i r e s a s e l e c t e d a s b e s t o s f i b e r g r a d e ( o r r a n g e o f g r a d e s ). T a b l e 7 l i s t s v a r i o u s t y p e s o f c h r y s o t i l e a p p l i c a t i o n s i n r e l a t i o n t o Q S f i b e r g r a d e s. 11. ALTERNATIVE INDUSTRIAL FIBERS AND MATERIALS C o n s i d e r a b l e e f f o r t h a s b e e n d e v o t e d t o f i n d i n g a l t e r n a t i v e f i b e r s o r m i n e r a l s t o r e p l a c e a s b e s t o s f i b e r s i n t h e i r a p p l i c a t i o n s. S u c h efforts have been motivated by various reasons, typically, availability and cost, and more recently, health and liability concerns. D u r i n g W o r l d W a r I I, s o m e c o u n t r i e s l o s t a c c e s s t o a s b e s t o s f i b e r s u p p l i e s a n d h a d t o d e v e l o p s u b s t i t u t e m a t e r i a l s. A l s o, i n t h e p r o d u c t i o n o f f i b e r - r e i n f o r c e d c e m e n t p r o d u c t s, m a n y d e v e l o p i n g c o u n t r i e s f o c u s e d o n r e a d i l y a v a i l a b l e, l o w - c o s t c e l l u l o s e f i b e r s a s a n a l t e r n a t i v e t o a s b e s t o s f i b e r s. S i n c e t h e s, h o w e v e r, s y s t e m a t i c r e s e a r c h h a s b e e n p u r s u e d i n s e v e r a l i n d u s t r i a l i z e d c o u n t r i e s t o r e p l a c e a s b e s t o s f i b e r s i n m a n y c u r r e n t a p p l i c a t i o n s b e c a u s e o f p e r c e i v e d h e a l t h r i s k s. T h e s u b s t i t u t i o n o f a s b e s t o s f i b e r s b y o t h e r t y p e s o f f i b e r s o r m i n e r a l s m u s t, i n p r i n c i p l e, c o m p l y w i t h t h r e e t y p e s o f c r i t e r i a ( 4 0 ) : t h e t e c h n i c a l f e a s i b i l i t y o f t h e s u b s t i t u t i o n ; t h e g a i n i n t h e s a f e t y o f t h e a s b e s t o s - f r e e p r o d u c t r e l a t i v e t o t h e a s b e s t o s - c o n t a i n i n g p r o d u c t ; a n d t h e a v a i l a b i l i t y o f t h e s u b s t i t u t e a n d i t s c o m p a r a t i v e c o s t. I n s o m e a p p l i c a t i o n s, p a r t i c u l a r l y t h o s e t h a t r e l y o n s e v e r a l c h a r a c t e r i s t i c f e a t u r e s o f a s b e s t o s f i b e r s, t h e s u b s t i t u t i o n h a s p r e s e n t e d a s i g n i f i c a n t c h a l l e n g e. F o r e x a m p l e, i n f i b e r - c e m e n t c o m p o s i t e s, t h e f i b e r s m u s t e x h i b i t h i g h t e n s i l e s t r e n g t h, g o o d d i s p e r s i o n i n 1 3

14 p o r t l a n d c e m e n t p a s t e s, a n d h i g h r e s i s t a n c e t o a l k a l i n e e n v i r o n m e n t s. L i k e w i s e, i n f r i c t i o n m a t e r i a l s, s e v e r a l p r o p e r t i e s o f t h e a s b e s t o s f i b e r s a r e i m p o r t a n t : h i g h a f f i n i t y t o w a r d r e s i n s, g o o d h e a t r e s i s t a n c e, h i g h f r i c t i o n c o e f f i c i e n t, a n d l o w a b r a s i o n o f t h e o p p o s i n g s u r f a c e. I n s u c h a p p l i c a t i o n s, t h e r e p l a c e m e n t o f a s b e s t o s f i b e r s h a s r e q u i r e d a c o m b i n a t i o n o f s e v e r a l m a t e r i a l s. T a b l e 8 l i s t s s o m e o f t h e m a t e r i a l s a n d f i b e r s t h a t h a v e b e e n s u g g e s t e d o r u s e d i n t h e d e v e l o p m e n t o f a s b e s t o s - f r e e p r o d u c t s a l o n g w i t h a n e s t i m a t e o f t h e c o s t r a n g e s o f a s b e s t o s f i b e r s a n d s e v e r a l t y p e s o f s u b s t i t u t i o n m a t e r i a l s. V a r i o u s a s b e s t o s s u b s t i t u t i o n s t r a t e g i e s h a v e b e e n f o l l o w e d a n d a w i d e r a n g e o f a s b e s t o s - f r e e p r o d u c t s h a v e b e e n d e v e l o p e d. F o r e x a m p l e, i n b u l k t h e r m a l i n s u l a t i o n a n d s p r a y e d i n s u l a t i o n c o a t i n g a p p l i c a t i o n s, s y n t h e t i c m i n e r a l f i b e r s ( g l a s s o r s l a g f i b e r s ) a n d c e l l u l o s e f i b e r p r o d u c t s h a v e r e p l a c e d n a t u r a l a s b e s t o s f i b e r s. C l o t h i n g m a d e f r o m a r a m i d f i b e r s o r a l u m i n i z e d g l a s s f i b e r s h a s r e p l a c e d a s b e s t o s i n m o s t t e x t i l e s. A s b e s t o s i n f l o o r t i l e h a s b e e n r e p l a c e d b y a c o m b i n a t i o n o f s y n t h e t i c f i b e r s a n d n o n f i b r o u s f i l l e r s. A s b e s t o s p a c k i n g s h a v e b e e n r e p l a c e d b y v a r i o u s m a t e r i a l s i n c l u d i n g a r a m i d a n d g l a s s f i b e r, g r a p h i t e m i x t u r e s, a n d c e l l u l o s e f i b e r s. I n f i b e r - c e m e n t c o n s t r u c t i o n m a t e r i a l s, s e v e r a l a l t e r n a t i v e s a r e b e i n g p r a c t i c e d, e i t h e r u s i n g c e l l u l o s i c f i b r o u s p r o d u c t s o r s y n t h e t i c o r g a n i c f i b e r s s u c h a s p o l y p r o p y l e n e ( P P ) o r p o l y a c r y l o n i t r i l e o r a l t e r n a t i v e p r o d u c t s s u c h a s c a s t i r o n, P V C, o r P P p i p e. T h e u s e o f a s b e s t o s - c e m e n t p r o d u c t s s t i l l i s c o n s i d e r a b l e o w i n g t o l o w c o s t, a v a i l a b i l i t y, l o w t e c h n o l o g y r e q u i r e m e n t s, a n d p e r f o r m a n c e. F o r f r i c t i o n m a t e r i a l a p p l i c a t i o n s, c o m p o s i t e m a t e r i a l s c o m p r i s i n g g l a s s o r m e t a l l i c f i b e r s w i t h o t h e r m i n e r a l s h a v e b e e n d e v e l o p e d. I n t h e s e a p p l i c a t i o n s, a r a m i d a n d g r a p h i t e f i b e r s a r e e f f e c t i v e, a l t h o u g h t h e c o s t o f t h e s e m a t e r i a l s r e s t r i c t s t h e i r u s e t o h e a v y d u t y o r h i g h t e c h n o l o g y a p p l i c a t i o n s. S u b s t i t u t i o n h a s b e e n m o r e s u c c e s s f u l i n d i s k b r a k e p a d s a s c o m p a r e d w i t h b r a k e s h o e s ( 4 0 ). E f f o r t s t o s u b s t i t u t e f o r a s b e s t o s h a v e b e e n f a i r l y s u c c e s s f u l a s e v i d e n c e d b y t h e f a c t t h a t p e a k w o r l d a s b e s t o s p r o d u c t i o n w a s 4. 8 x t o n s i n , d e c l i n i n g t o 1. 9 x t o n s i n T h i s r e p r e s e n t s a d e c l i n e o f a b o u t 6 0 % i n t h e u s e o f a s b e s t o s d e s p i t e g r o w t h i n every market in which asbestos was used during the 25-year period. The availability of adequate replacement materials, the cost-top e r f o r m a n c e r a t i o o f s u c h m a t e r i a l s, a n d t h e u n c e r t a i n t y o f l o n g - t e r m h e a l t h r i s k s o f t h e s e r e p l a c e m e n t m a t e r i a l s h a v e l i m i t e d t h e e x t e n t o f s u b s t i t u t i o n o f a s b e s t o s f i b e r s b y o t h e r f i b e r s i n s o m e p r o d u c t a p p l i c a t i o n s. 12. HEALTH AND SAFETY T h e r e l a t i o n s h i p b e t w e e n w o r k p l a c e e x p o s u r e t o a i r b o r n e a s b e s t o s f i b e r s a n d r e s p i r a t o r y d i s e a s e s i s o n e o f t h e m o s t w i d e l y s t u d i e d s u b j e c t s o f m o d e r n e p i d e m i o l o g y ( ). A s b e s t o s - r e l a t e d h e a l t h c o n c e r n s w e r e f i r s t r a i s e d a t t h e b e g i n n i n g o f t h e c e n t u r y i n t h e U n i t e d K i n g d o m, w h i c h a p p e a r s t o h a v e b e e n t h e f i r s t c o u n t r y t o r e g u l a t e t h e a s b e s t o s - u s e r i n d u s t r y ( 4 4 ). I t w a s n t u n t i l t h e e a r l y s, h o w e v e r, t h a t, r e s e a r c h e r s f i r m l y e s t a b l i s h e d a c o r r e l a t i o n b e t w e e n w o r k e r e x c e s s e x p o s u r e t o a s b e s t o s f i b e r s a n d r e s p i r a t o r y c a n c e r d i s e a s e s ( 4 5 ). T h i s f i n d i n g t r i g g e r e d a s i g n i f i c a n t r e s e a r c h e f f o r t t o u n r a v e l i m p o r t a n t i s s u e s s u c h a s t h e i n f l u e n c e o f f i b e r s i z e, s h a p e, c r y s t a l s t r u c t u r e, a n d c h e m i c a l c o m p o s i t i o n ; t h e r e l a t i o n s h i p b e t w e e n e x p o s u r e l e v e l s a n d d i s e a s e s ; t h e c o n s e q u e n c e o f e x p o s u r e t o a s b e s t o s f i b e r s i n d i f f e r e n t t y p e s o f i n d u s t r i e s, o r f r o m d i f f e r e n t t y p e s o f p r o d u c t s ; a n d t h e d e v e l o p m e n t o f t e c h n o l o g i e s t o r e d u c e worker exposure. T h e r e s e a r c h e f f o r t s r e s u l t e d i n a c o n s e n s u s i n s o m e a r e a s, a l t h o u g h c o n t r o v e r s y s t i l l r e m a i n s i n o t h e r a r e a s. I t i s w i d e l y r e c o g n i z e d t h a t t h e i n h a l a t i o n o f l o n g ( c o n s i d e r e d u s u a l l y a s l o n g e r t h a n 5 :m ), t h i n, a n d d u r a b l e f i b e r s i n h i g h c o n c e n t r a t i o n s o v e r a l o n g p e r i o d o f t i m e c a n i n d u c e o r p r o m o t e l u n g c a n c e r. I t i s a l s o w i d e l y a c c e p t e d t h a t a s b e s t o s f i b e r s c a n b e a s s o c i a t e d w i t h t h r e e t y p e s o f d i s e a s e s ( 4 6 ) : a s b e s t o s i s : a l u n g f i b r o s i s r e s u l t i n g f r o m l o n g - t e r m, h i g h l e v e l e x p o s u r e s t o a i r b o r n e f i b e r s ; l u n g c a n c e r : u s u a l l y r e s u l t i n g f r o m l o n g - t e r m h i g h l e v e l e x p o s u r e s a n d o f t e n c o r r e l a t e d w i t h a s b e s t o s i s ; m e s o t h e l i o m a : a r a r e f o r m o f c a n c e r o f t h e l i n i n g ( m e s o t h e l i u m ) o f t h e t h o r a c i c a n d a b d o m i n a l c a v i t i e s. A f u r t h e r c o n s e n s u s d e v e l o p e d w i t h i n t h e s c i e n t i f i c c o m m u n i t y r e g a r d i n g t h e r e l a t i v e c a r c i n o g e n i c i t y o f t h e d i f f e r e n t t y p e s o f a s b e s t o s f i b e r s. T h e r e i s s t r o n g e v i d e n c e t h a t t h e g e n o t o x i c a n d c a r c i n o g e n i c p o t e n t i a l s o f a s b e s t o s f i b e r s a r e n o t i d e n t i c a l ; i n p a r t i c u l a r m e s o t h e l i a l c a n c e r i s m o s t s t r o n g l y a s s o c i a t e d w i t h a m p h i b o l e f i b e r s ( ). T h e r e p l a c e m e n t o f a s b e s t o s f i b e r s b y o t h e r f i b r o u s m a t e r i a l s h a s r a i s e d s i m i l a r h e a l t h i s s u e s a b o u t s u b s t i t u t e m a t e r i a l s. H o w e v e r, l u n g c a n c e r h a s a l a t e n c y p e r i o d o f a p p r o x i m a t e l y 2 5 y e a r s a n d f i b e r e x p o s u r e l e v e l s t o s u b s t i t u t e s a r e f a r l o w e r t h a n t h o s e t h a t p r e v a i l e d h a l f a c e n t u r y a g o w i t h a s b e s t o s. C o n s e q u e n t l y, t h e e p i d e m i o l o g i c a l d a t a o n m o s t s u b s t i t u t e s i s i n s u f f i c i e n t t o e s t a b l i s h s t a t i s t i c a l l y s i g n i f i c a n t c o r r e l a t i o n s b e t w e e n e x p o s u r e a n d p u l m o n a r y d i s e a s e ( 5 2 ). A p o s s i b l e e x c e p t i o n i s s l a g f i b e r s f o r w h i c h s e v e r a l s t u d i e s o n w o r k e r p o p u l a t i o n s a r e a v a i l a b l e o v e r e x t e n d e d p e r i o d s ( 5 3 ) ; s o m e r e s u l t s s h o w a s u b s t a n t i a l i n c r e a s e i n l u n g c a n c e r o c c u r r e n c e. C o n s e q u e n t l y, t h e t o x i c i t y o f a s b e s t o s s u b s t i t u t e f i b e r s r e m a i n s a s u b j e c t o f a c t i v e i n v e s t i g a t i o n. 13. REGULATION T h e i d e n t i f i c a t i o n o f h e a l t h r i s k s a s s o c i a t e d w i t h l o n g - t e r m, h i g h l e v e l e x p o s u r e t o a s b e s t o s f i b e r s, t o g e t h e r w i t h t h e f a c t t h a t l a r g e q u a n t i t i e s o f t h e s e m i n e r a l s w e r e u s e d ( s e v e r a l m i l l i o n t o n s a n n u a l l y ) i n a v a r i e t y o f a p p l i c a t i o n s, p r o m p t e d t h e e n a c t m e n t o f r e g u l a t i o n s t o l i m i t t h e m a x i m u m e x p o s u r e o f a i r b o r n e f i b e r s i n w o r k p l a c e e n v i r o n m e n t s. T h e e x p o s u r e l i m i t s m a y b e d e f i n e d a s average or peak values, measured either as a weight or as a number of fibers-per-unit-volume. The International Labor Organization a d o p t e d t h e f o l l o w i n g d e f i n i t i o n ( C o n v e n t i o n 1 6 2, a r t i c l e 2 d a n d R ) : t h e t e r m r e s p i r a b l e a s b e s t o s f i b e r s m e a n s a s b e s t o s f i b e r s h a v i n g a d i a m e t e r o f l e s s t h a n 3 :m and a length-to-diameter ratio greater than 3:1. Only fibers of a length greater than 5 :m s h a l l b e t a k e n i n t o a c c o u n t f o r p u r p o s e s o f m e a s u r e m e n t. I n g e n e r a l, e x p o s u r e l i m i t s v a r y c o n s i d e r a b l y a m o n g c o u n t r i e s w i t h t h e r a n g e b e i n g from 0.1 fibers per cubic centimeter (f/cc) for 4- or 8-hour time-weighted-average exposure to 2 f/cc. 1 4

15 I n a c c o r d a n c e w i t h d e m o n s t r a t e d d i f f e r e n c e s b e t w e e n t h e v a r i o u s a s b e s t o s f i b e r t y p e s, w o r k p l a c e r e g u l a t i o n s i n m a n y c o u n t r i e s s p e c i f y d i f f e r e n t e x p o s u r e l i m i t s f o r c h r y s o t i l e a n d t h e a m p h i b o l e s ( 5 4 ). T y p i c a l l y, e x p o s u r e l i m i t s f o r a m p h i b o l e a s b e s t o s a r e o n e - h a l f t o o n e - t e n t h t h a t f o r c h r y s o t i l e. A l s o, t h e E u r o p e a n U n i o n v o t e d t o b a n t h e u s e o f a m p h i b o l e a s b e s t o s a n d i t s u s e h a s b e e n d i s c o n t i n u e d. M o r e o v e r, t o a l l e v i a t e e s t a b l i s h e d, o r a p p r e h e n d e d, r i s k f r o m s u b s t i t u t e f i b e r s, r e g u l a t i o n s o f t e n s p e c i f y m a x i m u m e x p o s u r e l i m i t s f o r s y n t h e t i c f i b e r s ( 5 5 ). S o m e c o u n t r i e s h a v e o p t e d f o r a b r o a d e r a p p r o a c h a n d h a v e a d o p t e d r e g u l a t i o n s t h a t a l s o m i n i m i z e e x p o s u r e o f t h e g e n e r a l p u b l i c t o e n v i r o n m e n t a l a s b e s t o s f i b e r s, i. e., b y b a n n i n g o r r e s t r i c t i n g a s b e s t o s i m p o r t s a n d t y p e s o f a p p l i c a t i o n s. C o u n t r i e s t h a t h a v e b a n n e d ( e i t h e r a c o m p l e t e b a n s o r a b a n w i t h e x e m p t i o n s ) o r a r e p h a s i n g o u t t h e u s e o f a s b e s t o s a n d i n s o m e c a s e s, a s b e s t o s p r o d u c t s, i n t h e next few years include Argentina, Austria, Belgium, Chile, Denmark, Finland, France, German, Italy, Netherlands, Norway, Poland, S a u d i A r a b i a, S w e d e n, S w i t z e r l a n d, a n d t h e U n i t e d K i n g d o m. T h e E u r o p e a n U n i o n v o t e d t o b a n a s b e s t o s u s e i n m o s t a p p l i c a t i o n s b y i t s m e m b e r s b y T h e o b v i o u s t r e n d w i t h a s b e s t o s i s t o w a r d s t r i c t e r r e g u l a t i o n s o r t h e b a n n i n g o f i t s u s e. B a n n i n g h a s o c c u r r e d f o r t h e m o s t p a r t i n d e v e l o p e d c o u n t r i e s w h e r e s u b s t i t u t e m a t e r i a l s o r a l t e r n a t i v e p r o d u c t s a r e r e a d i l y a v a i l a b l e a n d i t i s e c o n o m i c a l l y f e a s i b l e t o u s e a s b e s t o s s u b s t i t u t e s. I n l e s s e r - d e v e l o p e d c o u n t r i e s w h e r e e c o n o m i c s a n d t h e l e v e l o f i n d u s t r i a l d e v e l o p m e n t i s a f a c t o r, a s b e s t o s s u b s t i t u t e s a r e n o t y e t c o n s i d e r e d t o b e a s u i t a b l e o p t i o n i n m a n y c a s e s. D e s p i t e t h e c u r r e n t b a n s a n d c o n t i n u e d o p p o s i t i o n t o t h e u s e o f a s b e s t o s, m a r k e t s f o r a s b e s t o s p r o b a b l y w i l l e x i s t l o n g i n t o t h e f u t u r e. C o n s u m p t i o n c a n b e e x p e c t e d t o d e c l i n e a s s u b s t i t u t e s a n d a l t e r n a t i v e p r o d u c t s g a i n f a v o r i n t h e r e m a i n i n g w o r l d m a r k e t s. T h i s, h o w e v e r, i s a p r o c e s s t h a t p r o b a b l y w i l l o c c u r o v e r a p e r i o d o f d e c a d e s. E v e n t h e n, t h e r e p r o b a b l y w i l l r e m a i n s p e c i a l i z e d a p p l i c a t i o n s f o r a s b e s t o s, p a r t i c u l a r l y f o r m a t r i x - b a s e d p r o d u c t s. BIBLIOGRAPHY A s b e s t o s i n ECT 1st ed., Vol. 2, pp , by M. S. Badollet, Johns-Manville Research Center; in ECT 2 n d e d., V o l. 2, p p b y M. S. B a d o l l e t, C o n s u l t a n t ; i n ECT 3rd ed., Vol. 3, pp , by W. C. Streib, Johns-Manville Corporation, in ECT 4 th ed., Vol. 3, pp , by C.R. Jolicoeur, J. Alary, and A. Sokov, Universite de Sherbrooke, Canada. 1. M. Ross, R. A. Kuntze, and R. A. Clifton, in B. Levadie, ed., Definition for Asbestos and Other Health Related Silicates, A S T M S T P 8 3 4, A m e r i c a n S o c i e t y f o r T e s t i n g a n d M a t e r i a l s, P h i l a d e l p h i a, P a., , p W. J. Campbell and co-workers, Selected Silicates Minerals and Their Asbestiform Varieties, I C , U. S. B u r e a u o f M i n e s, W a s h i n g t o n, D. C., , p , A. A. Hodgson, Scientific Advances in Asbestos, 1967 to 1985, A n j a l e n a P u b l i c a t i o n, C r o w t h o r n e, U K, , p. 1 0, 1 4, 1 7, 2 3, 4 2, 5 3, 7 6, 7 8, 8 5, , H. C. W. Skinner, M. Ross, and C. Frondel, Asbestos and Other Fibrous Materials, Oxford Press, New York, 1988, p , 25, 3 1, 3 4, 3 5, A. Europaeus-Äyräpää, Acta Archaeol. 1, ( ). 6. J. Alleman and B. Mossman, Scientific American, July 1997, p O. B o w l e s, A s b e s t o s T h e S i l k o f t h e M i n e r a l K i n g d o m, T h e R u b e r o i d C o., N e w Y o r k, , p M. A. Bernarde, ed., Asbestos: The Hazardous Fiber, C R C P r e s s, B o c a R a t o n, F l a., , p. 4, , 4 1, R. L. Virta, Asbestos, U.S. Geological Survey Minerals Yearbook 2000 preprint, 2001, 7 p E. J. W. Whittaker, in R. L. Ledoux, ed., Short Course in Mineralogical Techniques of Asbestos Determination, V o l. 4, Mineralogical Association of Canada, Quebec, Canada, May 1979, p. 16, 24-28, Report of the Royal Commission on Matters of Health and Safety Arising from the Use of Asbestos in Ontario, J. S. Dupré, c h a i r m a n, V o l. 1, O n t a r i o G o v e r n m e n t B o o k - S t o r e, T o r o n t o, O n t a r i o, C a n a d a, , p. 7 8, F. J. W i c k s, i n R e f. 1 0, p. 5 8, Asbestos Factbook, A s b e s t o s, W i l l o w G r o v e, P a., E. J. W. Whittaker, Acta Crystallogr. 1 0, ( ) K. Yada, Acta Crystallogr. 2 7, ( ) J. F. A l a r y, Y. C ô t é, a n d C. J o l i c o e u r, u n p u b l i s h e d d a t a, C. J o l i c o e u r a n d D. D u c h e s n e, Can. J. Chem. 5 9 ( 1 0 ), ( ) E. P a p i r e r a n d P. R o l a n d, Clays and Clay Miner. 2 9, ( ) E. Orowan, Rep. Prog. Phys. 1 2, ( ) R. G. Bryans and B. Lincoln, Paper 6.24, 3rd International Conference on the Physics and Chemistry of Asbestos Minerals, Q u é b e c, C a n a d a, R. G. Bryans and B. Lincoln, Paper 3.6, 2nd International Conference on the Physics and Chemistry of Asbestos Minerals, U n i v e r s i t é d e L o u v a i n, L o u v a i n, B e l g i u m, A. A. Hodgson, in L. Michaels and S. S. Chissick, eds., Asbestos: Properties, Applications and Hazards, Vol. 1, John Wiley & Sons, Inc., New York, 1979, p , S. Speil and J. P. Leinerveber, Environ. Res. 2 ( 3 ), ( ). 1 5

16 2 4. G. Stroink, R. A. Dunlop, and D. Hutt, Can. Mineral. 1 9, ( ) A. A. Hodgson and C. A. White, Papers 2-10, The Physics and Chemistry of Asbestos Minerals, Oxford Conference on Asbestos Minerals, Oxford, UK, Z. Johan and co-workers, Adv. Org. Geochem., p ( ) T. Otouma and S. Take, Paper 5.21, i n R e f L. Zapata and co-workers, Paper 3.4, i n R e f U. S. P a t. 4, 3 5 6, ( O c t. 2 6, ), J. M. L a l a n c e t t e a n d J. D u n n i g a n ( t o S o c i é t é N a t i o n a l e d e L ' a m i a n t e ) E. Papirer, Clays and Clay Miner. 2 9, 6 9 ( ) U.S. Pat. 3,689,430 (Sept. 5, 1972), P. C. Yates (to E. I. du Pont de Nemours & Co., Inc.) W. C. McCrone, The Asbestos Particle Atlas, A n n A r b o r S c i e n c e P u b l i s h e r s, A n n A r b o r, M i c h., , p F. A. Settle, ed., Handbook of Instrumental Techniques for Analytical Chemistry, Prentice-Hall, New Jersey, 1997, 995 p M. J. Keane et al., J. Toxicology and Environmental Health, 57(8), p (1994) M. Cossette and P. Delvaux, in Ref. 7, p An Appraisal of Minerals Availability for 34 Commodities, U. S. B u r e a u o f M i n e s B u l l e t i n 6 9 2, W a s h i n g t o n, D. C., , p Chrysotile Asbestos Test Manual, 3 r d e d., A s b e s t o s T e x t i l e I n s t i t u t e a n d t h e Q u e b e c A s b e s t o s M i n i n g A s s o c i a t i o n, Q u e b e c, C a n a d a, ( r e v i s e d ) The Economics of Asbestos, 6 t h e d., R o s k i l l I n f o r m a t i o n S e r v i c e s L t d., L o n d o n, , p J. Harrod and V. Thorpe, Asbestos, Politics and Economics of a Lethal Product, I n t e r n a t i o n a l F e d e r a t i o n o f C h e m i c a l, E n e r g y and General Workers' Unions, Geneva, Switzerland, A. A. Hodgson, ed., Alternatives to Asbestos, The Pros and Cons, John Wiley & Sons, Inc., New York, 1989, p. xi Asbestos and Other Natural Mineral Fibers, WHO Environmental Health Criteria 53, World Health Organization, Geneva, S w i t z e r l a n d, J. Bignon, J. Peto, and R. Saracci, eds., Nonoccupational Exposure to Mineral Fibres, I A R C S c i e n t i f i c P u b l i c a t i o n N o. 9 0, International Agency for Research on Cancer, Lyon, France, Occupational Exposure Limit for Asbestos, Report of a WHO Meeting, Oxford, UK, Apr , R. Murray, Br. J. Ind. Med. 4 7, ( ) I. J. Selikoff, J. Churg, and E. C. Hammond, J. Am. Med. Assoc , ( ) J. Peto and co-workers, Environ. Health Perspect. 7 0, 5 1 ( ) Robert Nolan et al., eds., The Health Effects of Chrysotile Asbestos, Special Publication 5, T h e C a n a d i a n M i n e r a l o g i s t, O t t a w a, O n t a r i o, M. J. Gardner and C. A. Powell, J. Soc. Occup. Med. 3 6 ( 4 ), ( ) R. W i l s o n a n d B. P r i c e, paper 5.3, i n R e f Graham Gibbs, paper 4.1, i n R e f John Dement, Carcinogenicity of Asbestos-Differences by Fiber Type?, presented at 2001 Asbestos Health Effects Conference, May 24-25, 2001, Oakland, CA, Environmental Protection Agency M. Camus, paper 4.8, i n R e f I n t e r n a t i o n a l S y m p o s i u m o n M a n - m a d e M i n e r a l F i b e r s i n t h e W o r k i n g E n v i r o n m e n t, C o p e n h a g e n, D e n m a r k, O c t , , Ann. Occup. Hyg. 3 1, 4 B ( ) Summary of Main Features of Asbestos/Health Regulations at the Workplace, AIA Information Memorandum (AIM) No. 3-80, A s b e s t o s I n t e r n a t i o n a l A s s o c i a t i o n, E p s o n, S u r r e y, U K, N o v Safety in the Use of Mineral and Synthetic Fibers, Working Documents (Part 1) of ILO Meeting of Experts, Geneva, Switzerland, A p r , , I n t e r n a t i o n a l L a b o r O r g a n i z a t i o n, , p

17 T a b l e 1. A s b e s t o s F i b e r P r o d u c t i o n M i n e r a l s p e c i e s a O t h e r d e s i g n a t i o n s M a j o r s o u r c e s b W o r l d p r o d u c t i o n, , % chrysotile w h i t e a s b e s t o s R u s s i a, C a n a d a, C h i n a, B r a z i l, Kazakhstan, and Zimbabwe > 9 9 % c u m m i n g t o n i t e g r u n e r i t e amosite (brown a s b e s t o s ) d e p o s i t s i n S o u t h A f r i c a n o n e riebeckite c r o c i d o l i t e ( b l u e a s b e s t o s ) d e p o s i t s i n S o u t h A f r i c a n o n e a n t h o p h y l l i t e d e p o s i t s i n B u l g a r i a, F i n l a n d, I n d i a, S o u t h A f r i c a, U n i t e d S t a t e s i n s i g n i f i c a n t a c t i n o l i t e d e p o s i t s i n S o u t h A f r i c a a n d T a i w a n i n s i g n i f i c a n t t r e m o l i t e d e p o s i t s i n I n d i a, I t a l y, K o r e a, P a k i s t a n, S o u t h A f r i c a i n s i g n i f i c a n t a C h r y s o t i l e i s i n t h e s e r p e n t i n e m i n e r a l g r o u p ; a l l o t h e r s a r e a m p h i b o l e s. b In order of production only for chrysotile. T a b l e 2. G e o l o g i c a l O c c u r r e n c e o f A s b e s t o s F i b e r s Chrysotile A m o s i t e [ ] a [ ] C r o c i d o l i t e [ ] T r e m o l i t e [ ] m i n e r a l s p e c i e s chrysotile c u m m i n g t o n i t e g r u n e r i t e riebeckite t r e m o l i t e s t r u c t u r e a s v e i n s i n s e r p e n t i n e a n d m a s s f i b e r d e p o s i t s l a m e l l a r, c o a r s e t o f i n e, f i b r o u s a n d a s b e s t i f o r m f i b r o u s i n i r o n s t o n e s l o n g, p r i s m a t i c, a n d fibrous aggregates o r i g i n a l t e r a t i o n a n d m e t a m o r p h i s m o f b a s i c i g n e o u s r o c k s r i c h i n m a g n e s i u m s i l i c a t e s m e t a m o r p h i c r e g i o n a l m e t a m o r p h i s m m e t a m o r p h i c e s s e n t i a l c o m p o s i t i o n h y d r o u s s i l i c a t e s o f magnesia hydroxy silicate of Fe and Mg hydroxy silicate of Na, Mg, and Fe hydroxy silicate of Ca and Mg a Chemical Abstract Service Registry numbers in square brackets. 1 7

18 T a b l e 3. E l e m e n t a l A n a l y s i s o f A s b e s t o s F i b e r s a, percentage Variety and source S i O 2 ( s i l i c a ) F e O ( f e r r o u s o x i d e ) Fe 2 O 3 (ferric o x i d e ) Al 2 O 3 ( a l u m i n a ) MgO (magnesia) C a O ( l i m e ) M n O (manganese o x i d e ) Na 2 O ( s o d i u m o x i d e ) K 2 O ( p o t a s s i u m o x i d e ) H 2 O, a d s o r b e d H 2 O +, c o m b i n e d Chrysotile Quebec Zimbabwe U r a l M t Crocidolite C a p e P r o v i n c e A u s t r a l i a B o l i v i a trace trace trace Amosite T r a n s v a a l Tremolite P a k i s t a n a R e f. 1 3.

19 T a b l e 4. P h y s i c a l a n d C h e m i c a l P r o p e r t i e s o f A s b e s t o s F i b e r s Chrysotile A m o s i t e C r o c i d o l i t e T r e m o l i t e c o l o r usually white to grayish green; may have tan c o l o r a t i o n yellowish gray to dark brown c o b a l t b l u e t o l a v e n d e r b l u e gray-white, green, yellow, blue l u s t e r silky v i t r e o u s t o p e a r l y silky to dull silky h a r d n e s s, M o h s s p e c i f i c g r a v i t y o p t i c a l p r o p e r t i e s b i a x i a l p o s i t i v e p a r a l l e l e x t i n c t i o n b i a x i a l p o s i t i v e p a r a l l e l e x t i n c t i o n b i a x i a l o b l i q u e e x t i n c t i o n b i a x i a l n e g a t i v e o b l i q u e e x t i n c t i o n r e f r a c t i v e i n d e x f l e x i b i l i t y high fair f a i r t o g o o d poor,generally brittle texture silky, soft to harsh c o a r s e b u t s o m e w h a t p l i a b l e s o f t t o h a r s h generally harsh s p i n n a b i l i t y very good fair fair p o o r t e n s i l e s t r e n g t h, M P a a < r e s i s t a n c e t o : a c i d s a l k a l i e s weak, undergoes fairly r a p i d a t t a c k very good fair, slowly attacked g o o d g o o d g o o d g o o d g o o d surface charge, mv ( z e t a p o t e n t i a l ) d e c o m p o s i t i o n temperature, C t o b -20 to r e s i d u a l p r o d u c t s f o r s t e r i t e, s i l i c a, eventually enstatite Fe and Mg pyroxenes, magnetite, hematite, s i l i c a Na and Fe pyroxenes, h e m a t i t e, s i l i c a Ca, Mg, and Fe pyroxenes, silica a T o c o n v e r t M p a t o p s i, m u l t i p l y b y b C h r y s o t i l e f i b e r s t e n d t o b e c o m e n e g a t i v e a f t e r w e a t h e r i n g a n d / o r l e a c h i n g. 1 9

20 T a b l e 5. W o r l d P r o d u c t i o n o f A s b e s t o s b y P r i n c i p a l P r o d u c i n g C o u n t r i e s, a,b, t o n s C o u n t r y B r a z i l , , , , , C a n a d a 1 4 6, , , , , 0 1 4, , 5 0 7, , 3 2 3, , , C h i n a , , , , , , S o u t h A f r i c a 5, , , , , , , , , U.S.S.R. c 4 9, , , , 0 6 5, , 0 7 0, , 4 0 0, , U n i t e d S t a t e s ( s o l d o r u s e d b y p r o d u c e r s ) 1, , , , , , , W 5, Zimbabwe 1 5, , , , , , , , , Other 5, , , , , , , , , T o t a l 1 7 5, , , , 2 9 0, , 2 1 0, , 4 9 0, , 7 0 0, , 0 1 0, , 0 7 0, a W o r l d t o t a l s, U. S. d a t a, a n d e s t i m a t e d d a t a a r e r o u n d e d t o t h r e e s i g n i f i c a n t d i g i t s ; m a y n o t a d d t o t o t a l s s h o w n. b W. W i t h h e l d. Z e r o. c S o v i e t R e p u b l i c s c o m b i n e d, t o ; R u s s i a, ; R u s s i a ( 7 5 0, t o n s ) a n d K a z a k h s t a n ( 2 3 3, t o n s ), Source: U.S. Geological Survey and U.S. Bureau of Mines Minerals Yearbook chapters on asbestos

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