Chapter 10 - B Identification of minerals with the petrographic microscope

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

Optic Sign and Biaxial Indicatrix

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

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

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

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

Conoscopic observation of interference figures Biaxial Biaxial

Conoscopic observation of interference figures Biaxial Biaxial Perpendicular to obtuse bisectrix Perpendicular to obtuse bisectrix

Conoscopic observation of interference figures Biaxial Biaxial Parallel to the axial plane Parallel to the axial plane

Conoscopic observation of interference figures Biaxial Biaxial Off-centre interference figures Off-centre interference figures

Conoscopic observation of interference figures Biaxial Biaxial Off-centre interference figures Off-centre interference figures

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

Quartz vs feldspar Extinction; twinning; cleavage; alteration Extinction; twinning; cleavage; alteration

Quartz vs feldspar Similarities: Similarities: Both low birefringence – gray to white interference colors Both low birefringence – gray to white interference colors Two most common rock-forming minerals and often occur together Two most common rock-forming minerals and often occur together Distinguishing features Distinguishing features FeatureQuartzFeldspar ExtinctionIrregular/patchy Extinction angle Twinning (Chapter 5) Rarely twinned Very commonly twinned CleavageNone Two good Optical character Uniaxial positive Biaxial K-feldspars: negative 2V small/large 2V small/large Plagioclase: positive/negative 2V large 2V large Alteration None; clear and colorless Alteration common – to mica Crystal form Irregular shape (anhedral) More regular, tabular shape (euhedral)

Alkali feldspar

Solid solution series: Solid solution series: Albite: NaAlSi 3 O 8 Albite: NaAlSi 3 O 8 Orthoclase: KAlSi 3 O 8 Orthoclase: KAlSi 3 O 8 Name Distinguishing features Orthoclase (low sanidine) Simple doublet twinning Biaxial negative Large 2V Microcline Cross-hatched twinning Biaxial negative Large 2V Low albite (also endmember of plagioclase (plutonic/metamorphic) Lamellar twinning Biaxial positive Very large 2V High albite (volcanic) Lamellar twinning Biaxial negative Moderate 2V (~45°) Sanidine (high sanidine) Distinguish from Qtz: Qtz – uniaxial positive Distinguish from Orthoclase – Small 2V Doublet twinning Biaxial negative Small 2V

Plagioclase

Plagioclase Solid solution Solid solution Albite: NaAlSi 3 O 8 Albite: NaAlSi 3 O 8 Anorthite: CaAl 2 Si 2 O 8 Anorthite: CaAl 2 Si 2 O 8 In between: Oligoclase, Andesine, Labradorite, Bytownite In between: Oligoclase, Andesine, Labradorite, Bytownite Ca:(Ca+Na) ratio NB for rock-classification Ca:(Ca+Na) ratio NB for rock-classification Use optical properties to estimate composition Use optical properties to estimate composition Extinction angles – varying angle with change in anorthite content Extinction angles – varying angle with change in anorthite content

Olivine

Olivine Solid solution Solid solution Forsterite Mg 2 SiO 4 Forsterite Mg 2 SiO 4 Fayalite Fe 2 SiO 4 Fayalite Fe 2 SiO 4 Biaxial (forsterite +; fayalite – ) Biaxial (forsterite +; fayalite – ) Large 2V Large 2V High relief relative to feldspar minerals High relief relative to feldspar minerals Pale green pleochroism Pale green pleochroism Unstable with quartz: olivine + quartz = orthopyroxene Unstable with quartz: olivine + quartz = orthopyroxene Med-High birefringence – second order Med-High birefringence – second order No or imperfect cleavage No or imperfect cleavage Commonly altered to serpentine, iddingsite and chlorite Commonly altered to serpentine, iddingsite and chlorite

Altered olivine Alteration to serpentine

Pyroxenes EnstatiteDiopside Aegirite/AegirineAugite

Amphiboles Hornblende

Pyroxenes and amphiboles Chain silicates Chain silicates Wide range of compositions Wide range of compositions Solid solutions between Mg, Fe, Ca, Na Solid solutions between Mg, Fe, Ca, Na Prismatic and elongated along c-axis Prismatic and elongated along c-axis PyroxenesSingle chain silicates PyroxenesSingle chain silicates Orthorhombic (orthopyroxenes) – (Mg, Fe) 2 Si 2 O 6 Monoclinic (clinopyroxenes – (Ca, Na, Mg, Fe) 2 Si 2 O 6 Most common: diopside 2 very good cleavages ~ perpendicular (92°-93°) Weak pleochroism AmphibolesDouble chain silicates AmphibolesDouble chain silicates Mostly monoclinic (Mg,Fe,Al,Na,Ca,K) 7 Si 8 O 22 (OH) 2 Anthophyllite – orthorhombic (Mg, Fe) 7 Si 8 O 22 (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 Lower birefringence (and therefore lower order interference colors) than olivine

Micas

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

Calcite

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

Epidote

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