Chapter 10 - B Identification of minerals with the petrographic microscope

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