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Camosun College GEOS 250 Lectures: 9:30-10:20 M T Th F300 Lab: 9:30-12:20 W F300

Introduction to Mineralogy Dr. Tark Hamilton Chapter 10: Lecture 30-32 Crystal Growth, Twinning, Defects, Colour & Magnetism. Camosun College GEOS 250 Lectures: 9:30-10:20 M T Th F300 Lab: 9:30-12:20 W F300. 6 foot Stalactites & Helictites Huw Cordey, Lechuguilla Cave, NM. White Sands NM

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Camosun College GEOS 250 Lectures: 9:30-10:20 M T Th F300 Lab: 9:30-12:20 W F300

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  1. Introduction to MineralogyDr. Tark HamiltonChapter 10: Lecture 30-32Crystal Growth, Twinning, Defects, Colour & Magnetism Camosun College GEOS 250 Lectures: 9:30-10:20 M T Th F300 Lab: 9:30-12:20 W F300

  2. 6 foot Stalactites & Helictites Huw Cordey, Lechuguilla Cave, NM White Sands NM Larry Fellows Satin Spar, Barry Marsh Alabaster Carvings, Tom Joe Gypsum ?

  3. Rob Lavinsky Martins da Pedra Eyre Peninsula, S. Aus. Gypsum: acicular & tabular S. Australia Desert Rose Bahia Argentina, M. Olsina Rob Lavinsky Crystals = Selenite

  4. (001) (120) (010) Gypsum - CaSO4•2H2O Variety.(Selenite) 2/m Left: Hourglass Tw [010](010)(110) Right: (010)(001) Fishtail Tw South Australia Rob Lavinsky (Twin Composition Plane = (100)

  5. Nucleus of 62 formula units of NaClCrystals React Through Their Exterior Surfaces Exterior ions Octahedral & Unsatisfied Anisodesmic Large Free Surface Energy Chemical Activity Large & Ready to PPT Or Dissolve Interior ions Octahedral & Satisfied Isodesmic Bond energy Chemical Activity = 1 fig_10_01

  6. Surface Growth & Reactivity of CrystalsDepends Upon Unsatisfied Charges Free Corner & Edges Likeliest face to grow: Corner > Step > Terrace Surface Clusters likely to redissolve Free Surface Energy ≈ Surface Area/Volume Interior Satisfied Ions fig_10_02

  7. Common Forms are Lattice Planeswith high Site Density e.g.: AB, AC, AD 0.7071 Filled Sites/Length = Site Density Points along Hypotenuse / Length = √ 0.2774 0.4472 0.3153 1.0000 fig_10_03

  8. Crystal Forms,Variable Growth Rates, - Vectoral Properties Vectoral Properties Depend on Direction: Hardness, Conductivity, Speed of light, Xray-diffraction (111) Alternate Na+ & Cl- planes (100) Equal Na+ & Cl- fig_10_04

  9. Discontinuous Vectoral Properties: • Pertain only to certain planes or directions • No intermediate values • Cleavage • Fracture • Rate of Growth • Rate of Solution • Chemical or Ionic Diffusion

  10. Crystal Growth, Colour Zonation & Transformation of Forms Octahedral nucleus Cube Overgrowth fig_10_05

  11. Mn & Fe Oxy-hydroxide Dendrites:Solnhofen Limestone Bavaria,P. Andresen Dessication & Bedding Permeability Mn more easily oxidized & More easily precipitated than Fe Dip direction

  12. Point, Line & Mosaic Defectsin a Hexagonal closest Packed Layer of Spheres Bad 737 Tail Good tool fig_10_07

  13. Point Defects • Point Defects Represent disorder, vacancy, Higher Temperature locations in structure • Shottky Defects: Cations (or Anions) absent from structure • Frenkel Defects: Dislocation of a Cation or Anion into an adjacent (normally vacant) site • Shottky & Frenkel Defects don’t affect Stoichiometry • Impurity Defects: Interstitial or Substitutional can affect colour even at ppb or ppm levels e.g. Ti in Quartz to form Amythest or Rutillated form

  14. Crystal Defects: Point, Line & Plane Frenkel Edge Dislocation Lineage Structure Impurity Defect Screw Dislocation Shottky fig_10_08

  15. Other Defects in Crystals • Stacking Faults: AB-AB-A –AB in Hexagonal, ABC-ABC- BC-ABC in Cubic, TOT-TOT-T-TOT in clays (a missing layer) • Omission solid solution: A more highly charged cation substitutes for 2 cations leaving 1 void as in 2K+AlSi3O8Pb+2AlSi3O8 + □AlSi3O8□Ca2Mg5Si8O22(OH)2Na+Ca2Mg5Al3+Si7O22(OH)2 Also in Beryl, Zeolites & in defect structures like Pyrrhotite (Fe2+1-3xFe3+2x)□x S Fe6S7 – Fe11S12 • Colour Centers: electron for anion as in Fluorite • Chain width errors in amphiboles, clays: curls

  16. Chain Width Errors in Inosilicates HRTEM photo: a٭=asinβ Cl = 56° & 124° fig_10_10

  17. Epitaxial Overgrowths (Energy) (oriented mineral-mineral contacts) Controls on Exsolution & Twinning Also granoblastic textures: Quartzite, Marble & foliations in schist, gneiss also Catalytic surfaces, templates, adsorbtion fig_10_11

  18. Parallel Crystal Growth (C-axes)(Really all one crystal) Scepter Quartz Barite Tablets fig_10_12

  19. Twinning: Symmetrical intergrowth of 2 or more crystals of the same mineral Twin Laws have a Composition Plane & or a rotational axis Or mirror, as a single extra symmetry element Atoms in the Composition Plane Fit both crystal lattices fig_10_13

  20. 3 Causes for Twinning • Growth twins are the interruption or change in the lattice during formation due to deformation from a larger substituting ion • Annealing or Transformation twins result of a change in crystal system during cooling as one form becomes unstable & the crystal structure re-organizes into a more stable form • Deformation or gliding twins result from stress on the crystal after the crystal has formed, as during regional metamorphism • HCP structure is the most likely to twin of the three common crystal structures: BCC, FCC, and HCP • Epitaxis and Parallel growth simply reduces free surface energy and is not twinning

  21. Contact (CP) & Penetration Twins Spinel Law CP=(111) Japan Law CP=(1122) Pyrite iron cross TA=[001] Fluorite TA=[111] Carlsbad Law TA=[001] CP=(010) fig_10_14

  22. Polysynthetic: multiple parallel twins Plagioclase Rhomb-Diagonal Perfect (001) (1012) Good (010) Labradoresence Carlsbad-albite Albite CP=(031) CP=(011) TA={001} fig_10_16

  23. Striations from Polysynthetic Twinning Perfect (001) Plagioclase Albite twinning (Triclinic) also: Ala-a, Ala-b, Acline, Pericline Magnetite Striated on (110) by (111) CP=(010) Magnetite Oct-Dodec (111) twins Pyrite 2/m 3 Cube (011) twins fig_10_17

  24. Common Twins of Monoclinic Minerals fig_10_19

  25. Orthorhombic Twin Laws PbCO3 CaCO3 Fairy Crosses Fe4Al16Si8O48H2 (monoclinic) fig_10_20

  26. (011) Tetragonal Twins (diagonal) TiO2 SnO2 fig_10_21

  27. Hexagonals twin most commonly Calcite C=TA Butterfly twin Calcite (0112) Negative rhombohedron Quartz twins: Brazil (1120) Dauphine (0001) Japan (1122) fig_10_22

  28. Spectrum & Causes of Mineral Colour fig_10_24

  29. Visible Absorbtion Near Infrared (Molecular Bands) Lattice Energy Visible & IR Spectrum of Be3Al2Si6O18 ± H2O,CO2 Transition metal ions (unfilled d orbitals) fig_10_25

  30. Crystal Field Splitting of d-electrons(promoted or demoted by octahedral anions) Axial promotion Toward anions dz2 Random Anions Planar demotion Between anions fig_10_26

  31. Absorbtion Spectra of 3 Gems Fe3Al2Si3O12 – Fe2+ 8fold (Mg,Fe)2SiO4 – Fe2+ 6fold BeAl2O4 – Fe 3+ 6fold fig_10_27

  32. Differential promotion of Cr-d electronsin Ruby versus Emerald Transmission Transmission Cr Absorbion Peaks In Ruby fig_10_28

  33. table_10_01

  34. Molecular Orbital TheoryExplains the Blue Colour in Sapphire Electron transfer ~ Vis+nearIR Fe2+A + Fe3+B Fe3+A + Fe3+B fig_10_29

  35. table_10_02

  36. Colour or f-centres in Purple Fluorite f = farbe “colour” in German fig_10_30

  37. Hole Colour Centres in Smoky Quartz Normal Quartz Al+H Substituted Quartz with Radiation damage (electron holes) e- has excited states fig_10_31

  38. table_10_03

  39. Physical Processes for Colour • Admixture or inclusions of other minerals/matter Green Quartz (Adventurine) – chlorite inclusions Black Calcite – graphite, MnOxides, petroleum Pink K-spar – Hematite inclusions Red Jasper – Hematite inclusions Feldspar (Sunstone) – Native Cu inclusions • Refraction of light for iridescence Feldspar – Labradorescence • Irradiation or heat treating Blue Topaz from Yellow Yellow Citrine from Smoky Quartz or Amythest Sapphire or Ruby from Corundum

  40. Fe2+ Fe3+2 O4 – Magnetite table_10_04

  41. Types of Magnetic Mineral Behaviour(in the presence of an external field) • Diamagnetic: paired electrons, no moment, repelled by field. e.g. Calcite, Quartz, Feldspar • Paramagnetic: few unpaired electrons, weakly attracted, thermal randomization dominates. e.g. Olivine, Augite, Hypersthene, Hornblende • Ferromagnetic: dominantly aligned unpaired electron domains. Strongly attracted & capable of remnance. e.g. Taenite & Kamacite in FeNi • Ferrimagnetic: aligned unpaired electons outweigh anti-aligned ones. e.g. Magnetite, Chromite, Pyrrhotite, Greigite, Smythite

  42. Unmagnetized & Magnetizedgranular multidomain magnetite Random no magnetic moment Alternate directions For grains > 10μ fig_10_32

  43. Taenite Native Fe0 Curie Point 770°C Magnetite Fe3+IV (Fe2+ Fe3+)VI O4 Curie Point 580°C fig_10_33

  44. Amorphous Alloy (disordered) Crystalline Alloy (ordered) fig_10_34

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