Biaxial minerals (cont.)

Transcription

1 Biaxial minerals (cont.) Extinction angles: Extinction in a biaxial mineral may be parallel (extinction angle = 0 relative to a prominent crystal face or cleavage direction), symmetrical (relative to 2 prominent cleavage directions), or oblique (extinction is at an angle to a prominent crystal face or cleavage direction) cleavage Vibration directions Parallel extinction Inclined extinction Symmetrical extinction Pleochroism: Biaxial minerals may show pleochroism with different transmission colors for the three different vibration directions. Recall: pleochroism can only be determined in plane polarized light. The pleochroic formula is the color of a mineral when each of the vibration directions is parallel to the lower polarizer (E-W), e.g., the pleochroic formula might be: X = blue, Y = light green, Z = dark green. To determine pleochroic formula, you need to know the optical orientation of the grain.

2 Twinning (a very brief discussion) Twins occur when two crystals of a given material are intergrown in such a way that all crystallographic directions of the first crystal unit are related to the corresponding directions of the second crystal unit by the operation of either (1) a pseudo mirror plane of symmetry, (2) a pseudo 2-fold axis of symmetry (much less frequently a 3-, 4-, or 6-fold axis), or (3) a pseudo center of symmetry. Classification by symmetry Reflection twins, e.g. Albite twins and Pericline twins in triclinic feldspars Rotation twins, e.g., Carlsbad twins in monoclinic and triclinic feldspars Centrosymmetric twins The plane of symmetry relating reflection twins is called the twin plane and the axis of symmetry relating rotation twins is called the twin axis. In reflection twins the twin plane is always parallel to a possible lattice plane (hkl). In rotation twins the twin axis is almost always 2- fold ; the direction of the axis may usually be specified by designating the zone axis [uvw] to which it is parallel or by the plane (hkl) to which it is perpendicular. Classification by spatial relationship If the two twinned individuals are in contact along a well-defined plane they are called contact twins and the plane of contact is called the composition plane. If the contact between the two twins is irregular they are called penetration twins. If the twin-producing symmetry operation is repeated numerous times along parallel planes, the twinning is called polysynthetic twinning (or multiple twinning). If the composition planes are at an angle to each other close to 120º, 90º or 60º, cyclic twins are produced.

3 Types of rotation-contact twins Normal twins [twin axis is normal to the composition plane, e.g., Albite twins]: specified as (hkl) Parallel twins and Complex twins [twin axis is parallel to the composition plane, e.g., Carlsbad twins]. In parallel twins the twin axis is parallel to a [uvw] direction (specified as [uvw]) while in complex twins the twin axis is perpendicular to a particular [uvw] direction specified as [uvw]/(hkl) read as the direction perpendicular to [uvw] within the plane (hkl). Symmetry-imposed restrictions A twin plane or axis can never coincide in direction with any symmetry operation (in the untwinned crystal) for which the twinning operation is identical (or a subgroup). For example, the (010) plane cannot be a twin plane in the isometric system because it is a mirror plane of symmetry. General Rule: In crystals that have a center of symmetry, twins can be related by a twin plane and/or by a 2-fold axis. Formation of twins Twins are commonly classified as (a) growth twin, (b) transformation twins or (3) gliding twins. Growth twins are primary as the name suggests and occur during the growth of the mineral during crystallization or recrystallization, e.g., most feldspar twins, pyroxene twins and amphibole twins. Transformation twins are secondary in that they are produced after the mineral is formed and they usually form during cooling as the structure transforms from a high symmetry state to a lower symmetry state, e.g., high quartz to low quartz transition at 573ºC. Mechanical twinning is commonly lamellar (multiple) and is a consequence of mechanical deformation, e.g., twinning in calcite.

4 Feldspar twinning Definitions: Twin Axis: Crystallographic direction about which a twin unit is rotated with respect to the other unit. Twin Plane: Crystallographic plane across which the two twin units show reflection symmetry (pseudo-mirror plane) Composition Plane: Surface on which two parts of a twin meet Normal twin: Twin axis is perpendicular to the composition plane. One twin unit is rotated 180º relative to the other twin unit Parallel twin: Twin axis lies in the composition plane and is // a crystallographic axis Common Twin laws Composition Plane Twin axis Twin type Normal twin laws: Albite (010) (010) Multiple (triclinic only) Manebach (001) (001) Simple Baveno (021) (021) Simple Parallel twin laws: Carlsbad (010) c simple (interpenetrant) Pericline Rhombic section b multiple (triclinic only)

5 Twinning in feldspars (cont.) 001) Albite twin Twin plane (010) Twin axis: (010) Usually polysynthetic Interpenetrant Carlsbad twin Twin axis: c axis

6 Twinning in feldspars (cont.)

7 Photomicrograph on left shows an olivine gabbro from the Skaergaard Intrusion (crossed polars). Plagioclase shows polysynthetic albite twinning (gray/white interference colors). Several of the plagioclase crystals show a combined albite-carlsbad twinning and a few show combined albite-pericline twinning. One grain shows Baveno/ Manebach twinning (rare). Olivine shows higher order interference colors. Jane Selverstone, University of New Mexico, 2003 The image above shows near-perfect albite twinning in plagioclase. The (001) cleavage is also visible

8 Albite+pericline twinning in microcline and anorthoclase (commonly called cross-hatched or tartan or grid twinning) Cross-hatched twinning in anorthoclase Cross-hatched twinning in microcline Carlsbad twinning in sanidine phenocryst

9 Determination of plagioclase compositions using the Michel-Levy method 1. Locate a plagioclase grain in which the (010) twin lamellae are in sharp focus [the (010) cleavage should also be sharp]. If both the (010) and (001) cleavages are sharp, you have an a-normal section (lucky you). 2. Rotate the grain until one set of albite twin lamellae are at extinction and record the angle on the stage goniometer. 3. Rotate the grain until the other set of twin lamellae are at extinction and record the angle on the stage goniometer. Extinction should be roughly symmetrical. 4. The extinction angle is the average of the two readings. 5. Repeat 1-4 for several additional grains and record the maximum reading. 6. From Fig in Nesse determine the composition of the feldspar making sure you use the correct curve. High: volcanic rocks. Low: plutonic rocks.

10 Multiple twinning in clinopyroxene Twinning and oscillatory zoning in plagioclase phenocryst, Mauna Kea Simple twin in a zoned amphibole from Mount Shasta, CA Hour-glass zoning in cpx phenocryst, Hawaii

11 Compositional zoning in minerals with extensive solid solution Zoned plagioclase feldspar, Mount Baker This is a false-colored BSE image of an oscillatory zoned plagioclase. Zoned plagioclases are common in volcanic and plutonic igneous rocks. Less common in metamorphic rocks. The image below (right) is a false-colored back scattered electron (BSE) image of an amphibole phenocryst in the Coleman Pinnacle flow, Mount Baker, WA. This crystal shows oscillatory zoning (the most common type of zoning in igneous rocks) The image (left) is a photomicrograph of a euhedral amphibole from Canary Islands. This type of zoning is called Sector zoning