Chapter 10 - B Identification of minerals with the petrographic microscope.
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Transcript of Chapter 10 - B Identification of minerals with the petrographic microscope.
![Page 1: Chapter 10 - B Identification of minerals with the petrographic microscope.](https://reader036.fdocuments.net/reader036/viewer/2022081513/56649f055503460f94c1a41a/html5/thumbnails/1.jpg)
Chapter 10 - B
Identification of minerals with the petrographic
microscope
![Page 2: Chapter 10 - B Identification of minerals with the petrographic microscope.](https://reader036.fdocuments.net/reader036/viewer/2022081513/56649f055503460f94c1a41a/html5/thumbnails/2.jpg)
Content
Conoscopic observation of interference figures Biaxial
Characteristics of important rock-forming minerals
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Optic Sign and Biaxial Indicatrix
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Conoscopic observation of interference figures
Biaxial Perpendicular to acute bisectrix
2V angle < 60°
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Conoscopic observation of interference figures
Biaxial Perpendicular to acute bisectrix
2V angle > 60° (but still < 90 °)
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Conoscopic observation of interference figures
Biaxial Perpendicular to optical axis
2V < 30°
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Conoscopic observation of interference figures
Biaxial Perpendicular to optical axis
2V > 30° (but still < 90 °)
![Page 8: Chapter 10 - B Identification of minerals with the petrographic microscope.](https://reader036.fdocuments.net/reader036/viewer/2022081513/56649f055503460f94c1a41a/html5/thumbnails/8.jpg)
Conoscopic observation of interference figures
Biaxial
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Conoscopic observation of interference figures
Biaxial Perpendicular to obtuse bisectrix
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Conoscopic observation of interference figures
Biaxial Parallel to the axial plane
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Conoscopic observation of interference figures
Biaxial Off-centre interference figures
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Conoscopic observation of interference figures
Biaxial Off-centre interference figures
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Characteristics of important rock-forming
minerals Quartz vs feldspar Alkali feldspars Plagioclase feldspars Olivine Pyroxenes and amphiboles Micas Calcite and dolomite Epidote
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Quartz vs feldspar Extinction; twinning; cleavage;
alteration
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Quartz vs feldspar Similarities:
Both low birefringence – gray to white interference colors
Two most common rock-forming minerals and often occur together
Distinguishing featuresFeature Quartz Feldspar
Extinction Irregular/patchy Extinction angle
Twinning (Chapter 5)
Rarely twinned Very commonly twinned
Cleavage None Two good
Optical character Uniaxial positive BiaxialK-feldspars: negative 2V small/largePlagioclase: positive/negative 2V large
Alteration None; clear and colorless
Alteration common – to mica
Crystal form Irregular shape (anhedral)
More regular, tabular shape (euhedral)
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Alkali feldspar
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Alkali feldspar
Solid solution series: Albite: NaAlSi3O8
Orthoclase: KAlSi3O8
Name Distinguishing features
Orthoclase (low sanidine) Simple doublet twinningBiaxial negativeLarge 2V
Microcline Cross-hatched twinningBiaxial negativeLarge 2V
Low albite (also endmember of plagioclase(plutonic/metamorphic)
Lamellar twinningBiaxial positiveVery large 2V
High albite (volcanic) Lamellar twinningBiaxial negativeModerate 2V (~45°)
Sanidine (high sanidine)Distinguish from Qtz: Qtz – uniaxial positiveDistinguish from Orthoclase – Small 2V
Doublet twinningBiaxial negativeSmall 2V
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Plagioclase
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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
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Olivine
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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
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Altered olivine
Alteration to serpentine
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Pyroxenes
EnstatiteDiopside
Aegirite/Aegirine Augite
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Amphiboles
Hornblende
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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: diopside2 very good cleavages ~ perpendicular
(92°-93°)Weak pleochroism
Amphiboles Double chain silicatesMostly 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
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Micas
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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
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Calcite
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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
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Epidote
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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