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Chapter 10 - B

Chapter 10 - B. Identification of minerals with the petrographic microscope. Content. Conoscopic observation of interference figures Biaxial Characteristics of important rock-forming minerals. Optic Sign and Biaxial Indicatrix. Conoscopic observation of interference figures. Biaxial

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Chapter 10 - B

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  1. Chapter 10 - B Identification of minerals with the petrographic microscope

  2. Content • Conoscopic observation of interference figures • Biaxial • Characteristics of important rock-forming minerals

  3. Optic Sign and Biaxial Indicatrix

  4. Conoscopic observation of interference figures • Biaxial • Perpendicular to acute bisectrix • 2V angle < 60°

  5. Conoscopic observation of interference figures • Biaxial • Perpendicular to acute bisectrix • 2V angle > 60° (but still < 90 °)

  6. Conoscopic observation of interference figures • Biaxial • Perpendicular to optical axis • 2V < 30°

  7. Conoscopic observation of interference figures • Biaxial • Perpendicular to optical axis • 2V > 30° (but still < 90 °)

  8. Conoscopic observation of interference figures • Biaxial

  9. Conoscopic observation of interference figures • Biaxial • Perpendicular to obtuse bisectrix

  10. Conoscopic observation of interference figures • Biaxial • Parallel to the axial plane

  11. Conoscopic observation of interference figures • Biaxial • Off-centre interference figures

  12. Conoscopic observation of interference figures • Biaxial • Off-centre interference figures

  13. Characteristics of important rock-forming minerals • Quartz vs feldspar • Alkali feldspars • Plagioclase feldspars • Olivine • Pyroxenes and amphiboles • Micas • Calcite and dolomite • Epidote

  14. Quartz vs feldspar • Extinction; twinning; cleavage; alteration

  15. Quartz vs feldspar • Similarities: • Both low birefringence – gray to white interference colors • Two most common rock-forming minerals and often occur together • Distinguishing features

  16. Alkali feldspar

  17. Alkali feldspar • Solid solution series: • Albite: NaAlSi3O8 • Orthoclase: KAlSi3O8

  18. Plagioclase

  19. Plagioclase • Solid solution • Albite: NaAlSi3O8 • Anorthite: CaAl2Si2O8 • In between: Oligoclase, Andesine, Labradorite, Bytownite • Ca:(Ca+Na) ratio NB for rock-classification • Use optical properties to estimate composition • Extinction angles – varying angle with change in anorthite content

  20. Olivine

  21. Olivine • Solid solution • Forsterite Mg2SiO4 • Fayalite Fe2SiO4 • Biaxial (forsterite +; fayalite – ) • Large 2V • High relief relative to feldspar minerals • Pale green pleochroism • Unstable with quartz: olivine + quartz = orthopyroxene • Med-High birefringence – second order • No or imperfect cleavage • Commonly altered to serpentine, iddingsite and chlorite

  22. Altered olivine Alteration to serpentine

  23. Pyroxenes Aegirite/Aegirine Augite Diopside Enstatite

  24. Amphiboles Hornblende

  25. Pyroxenes and amphiboles • Chain silicates • Wide range of compositions • Solid solutions between Mg, Fe, Ca, Na • Prismatic and elongated along c-axis • Pyroxenes Single chain silicates Orthorhombic (orthopyroxenes) – (Mg, Fe)2Si2O6 Monoclinic (clinopyroxenes – (Ca, Na, Mg, Fe)2Si2O6 Most common: diopside 2 very good cleavages ~ perpendicular (92°-93°) Weak pleochroism • Amphiboles Double chain silicates Mostly monoclinic (Mg,Fe,Al,Na,Ca,K)7Si8O22(OH)2 Anthophyllite – orthorhombic (Mg, Fe)7Si8O22(OH)2 Most common: hornblende 2 very good cleavages ~ 124° Distinctive blue (Na) or green (Ca) pleochroism • Lower birefringence (and therefore lower order interference colors) than olivine

  26. Micas

  27. Micas (and related chlorites) • Muscovite, biotite, lepidolite, chlorite • Excellent cleavage • Colored micas show distinctive pleochroism - biotite: green to brown • Optically biaxial negative • Platy habit • Very high birefringence • Chlorites lower birefringence than biotite and muscovite

  28. Calcite

  29. Calcite and dolomite (and aragonite) • Very high birefringence • Aragonite – biaxial • Calcite and dolomite • Uniaxial negative • Strong cleavage • Calcite – twinning at low angle to basal plane • Dolomite – twinning at high angle to basal plane

  30. Epidote

  31. Epidote • Orthosilicate mineral • Monoclinic • Resemble olivine, but: • Epidote displays cleavage • Epidote has weak pleochroism • Brilliant interference colors – off the chart for interference colors – due to abnormal dispersion

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