CEE 437 Lecture 10 Minerals and Mineral Properties. Thomas Doe

Transcription

1 CEE 437 Lecture 10 Minerals and Mineral Properties Thomas Doe

2 Outline Rock cycle Rock Forming Minerals Mineral Physical Properties Clay Minerals Microfractures and Fracture Mechanics

3 Crustal Composition Main Elemental Groups Silica Aluminum Ferro-Magnesian Ca, Na, K

4 Rock Cycle Crystallization at depth or extrusion at surface Igneous Rocks Magma Burial, metamorphism, recrystallization Melting Sediments Lithification Weathering, Erosion Sedimentary Rocks Metamorphic Rocks Burial, metamorphism, recrystallization

5 Mineral Differentiation Plate tectonics and Igneous Processes selective melting, selective recrystallization differentiation by density Weathering and Erosion Selective weathering Concentration of quartz (pure Si0 2 ) Conversion of alumino-silicates to clays Concentration of soluble residues in seawater Deposition Courser materials near sediment source Finer materials far from sediment source Redeposition of salts and solutes by evaporative (Na,KCl; CaSO 4 ) or biological processes (CaCO 3,; )

6 Subduction Zone Island Arc

7 Evolution of Continents North American Craton

8 Differentiation of Crustal Composition Weathering differentiating towards higher Silica Carbonate concentrated by organic processes Preferential melting of high-silica materials Concentration of C, Ca, Na, K in sea and air Original basaltic composition of crust

9 Convergent Margin - Continental

10 Bowen Reaction Series How to get many different rocks from one melt composition? Differentiation by selective crystallization and removal from system

11 Bowen s Reaction Series

12 Weathering Cycle

13 Elemental Fates Silicon tends to concentrate in crust quartz is very long lived Aluminum transforms from feldspars to clays Mica transform to clays Fe-Mg-Ca-Na-K concentrate in some clays and micas, concentrate in oceans in biosphere

14 Sedimentary Differentiation Sorting by Deposition Medium Sorting by Energy

15 Mineral Definition Naturally occurring material with unique combination of chemical composition and crystalline structure Natural non-minerals glasses, coal, amorphous silica Pseudomorphs: diamond:graphite

16 Mineral Groups Silicates: Major rock forming minerals Carbonates: Minor in igneous rocks, mainly found in organicderived rocks (limestones) and low-temperature cements and fracture fillings Dissolution relationships Sulfates (Gypsum) and Halides (salt, potash): Products of evaporation Extreme dissolution Oxides Weathering products Sulfides: Ores Late stages of igneous melts, reducing conditions Acid production, reactivity problems in concrete

17 Galena, PbS Graphite, C

18 Crystalline Structure of Calcite

19 Crystalline Symmetry Groups

20 Isomorphic Crystal Forms, Cubic System

21 Density Hardness Mineral Identification Color, luster (metallic, non-metalic, semimetallic) Crystalline habit Cleavage Optical microscopy Mineral chemistry, x-ray diffraction

22 Hardness Scale

23 X-Ray Diffraction Bragg s Law

24 Physical Properties Density (Gravity) Electrical Conductivity (Resisitivity) Thermal Expansion Strength Elasticity (Mechanical properties, Seismic/Acoustic Velocity Rheology (Plasticity,Viscosity)

25 Properties and Mineral Symmetry

26 Tensor Properties of Crystals Cubic Group Lower Symmetry Groups General Form for Heat Flow (for example)

27 Discussion: How to Rock Properties Relate to Mineral Structure How will anisotropy vary with crystal symmetry class? Rock Salt versus Quartz? How will aggregates of minerals (with same mineral behave? Cubic versus non cubic Rock fabric Material property contrasts

28 Rock Forming Minerals Composition of Crust Dominantly O, Si, Fe, Mg, Ca, Na, K Near surface importance of bio-processes Silicates from inorganic processes Carbonates mainly from shell-forming organisms

29 Crustal Composition Main Elemental Groups Silica Aluminum Ferro-Magnesian Ca, Na, K

30 Major Silicate Groups Silicon Tetrahedron separate tetrahedra olivine single chains pyroxene double chains amphibole sheet silicates micas and clays framework silicates feldspars (with Al substitution), quartz as pure silica

31 Silica Tetrahedron

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33 Forms of Silicates

34 Deformation Mechanisms

35 Effects on Physical Properties Anisotropy Properties differ by direction Heterogeneity Properties vary by location Mineral properties may have strong anisotropy when crystals are aligned Heterogeneity may have strong mechanical effects when different minerals have different deformation properties

36 Weathering Fates Feldspars to clays (clays, shales) Quartz endures (siltstones, sandstones) Calcium recirculated into carbonate minerals by organic processes (limestones) Consequence: Over time, evolution of less dense more silicic continental crust

37 Clay Minerals Extremely Important Mineral Group Seals Stability Pore pressure Chemical interaction Swelling Slaking Confusion as both Size and Mineral Classification

38 Clay Rock Cycle

39 Clay Viewed from Electron Microscope

40 Clay Sources Weathering Hydrothermal Alteration Deposition Clay Transformations Feldspar Illite Ferro-Magnesian Chlorite Volcanics (alkaline conditions) Smectite Volcanics (acidic conditions) Kaolinite Bentonite: plastic, highly swelling

41 Clay Units From West, Geology Applied to Engineering, Prentice Hall, 1995)

42 Two and Three-Layer Clay Structure From West, Geology Applied to Engineering, Prentice Hall, 1995)

43 Mixed Layer Clays From West, Geology Applied to Engineering, Prentice Hall, 1995)

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45 Minerals versus Rocks Minerals Elements Anisotropy from crystal structure Elastic Properties Thermal Properties Optical Properties Deformation Shear transformations Dislocations Rock Elements Intragranular Anisotropy from fabric Crystal anisotropy if preferred orientation Anisotropy from bedding, foliation, flow structures Intergranular Cements Microcracks Heterogeneity Mineral composition Other segregration processes

46 Translation of Mineral Properties to Rock Properties Comparison of mineral properties to rock properties Rocks have lower strength, especially tensile strength Anisotropy of minerals and heterogeneity of minerals Elasticity Thermal expansion Diversity of mineral orientation Creation of microcracks on mineral boundaries

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48 LEFM: Linear Elastic Fracture Mechanics Rocks are cracked materials LEFM represents alternative view of material strength that accounts well for scale and geometric effects Important in brittle materials Liberty Ships Comet passenger aircraft

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50 Stress Intensity: Y geometric factor σ stress a crack half length

51 Critical LEFM Elements Stress Intensity Concentration of stress at Crack Tips Fracture Toughness Measure of resistance to fracture Depends partly on plasticity of materials Lower in brittle materials

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