chapter 10 - b identification of minerals with the petrographic microscope
TRANSCRIPT
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 Perpendicular to acute bisectrix
2V angle < 60°
Conoscopic observation of interference figures
Biaxial Perpendicular to acute bisectrix
2V angle > 60° (but still < 90 °)
Conoscopic observation of interference figures
Biaxial Perpendicular to optical axis
2V < 30°
Conoscopic observation of interference figures
Biaxial Perpendicular to optical axis
2V > 30° (but still < 90 °)
Conoscopic observation of interference figures
Biaxial
Conoscopic observation of interference figures
Biaxial Perpendicular to obtuse bisectrix
Conoscopic observation of interference figures
Biaxial Parallel to the axial plane
Conoscopic observation of interference figures
Biaxial Off-centre interference figures
Conoscopic observation of interference figures
Biaxial Off-centre interference figures
Characteristics of important rock-forming
minerals Quartz vs feldspar Alkali feldspars Plagioclase feldspars Olivine Pyroxenes and amphiboles Micas Calcite and dolomite Epidote
Quartz vs feldspar Extinction; twinning; cleavage;
alteration
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)
Alkali feldspar
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
Plagioclase
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
Olivine
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
Altered olivine
Alteration to serpentine
Pyroxenes
EnstatiteDiopside
Aegirite/Aegirine Augite
Amphiboles
Hornblende
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
Micas
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
Calcite
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
Epidote
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