Post on 15-Dec-2015
Light Minerals: Quartz and Feldspars
Most acid igneous rocks are dominated by quartz and feldspar as the main minerals.Usually in granitic rocks there are two feldspars, one alkali and one plagioclase (albite).
Alkali FeldsparHand specimenPink colour most characteristicTranslucent/opaqueTwo sets of cleavagePearly lustreMonoclinic (except microcline)Thin sectionLow birefringenceLow reliefCleavage (two sets in sectionsPerpendicular to c)
Light Minerals: Quartz and Feldspars
Most acid igneous rocks are dominated by quartz and feldspar as the main minerals.Usually in granitic rocks there are two feldspars, one alkali and one plagioclase (albite).
Alkali FeldsparHand specimenPink colour most characteristicTranslucent/opaqueTwo sets of cleavagePearly lustreMonoclinic (except microcline)Thin sectionLow birefringenceLow reliefCleavage (two sets in sectionsPerpendicular to c)
QuartzHand specimenWhite, grey, yellowish whiteTranslucent to transparentNo cleavage but concoidal fractureVitreous lustreTrigonalThin sectionLow birefringenceLow reliefLack of cleavage
Light Minerals: Quartz and Feldspars
Most acid igneous rocks are dominated by quartz and feldspar as the main minerals.Usually in granitic rocks there are two feldspars, one alkali and one plagioclase (albite).
Alkali FeldsparHand specimenPink colour most characteristicTranslucent/opaqueTwo sets of cleavagePearly lustreMonoclinic (except microcline)Thin sectionLow birefringenceLow reliefCleavage (two sets in sectionsPerpendicular to c)
Alkali FeldsparHand specimenPink colour most characteristicTranslucent/opaqueTwo sets of cleavagePearly lustreMonoclinic (except microcline)Thin sectionLow birefringenceLow reliefCleavage (two sets in sectionsPerpendicular to c)
Light Minerals: Quartz and Feldspars
Most acid igneous rocks are dominated by quartz and feldspar as the main minerals.Usually in granitic rocks there are two feldspars, one alkali and one plagioclase (albite).
Alkali FeldsparHand specimenPink colour most characteristicTranslucent/opaqueTwo sets of cleavagePearly lustreMonoclinic (except microcline)Thin sectionLow birefringenceLow reliefCleavage (two sets in sectionsPerpendicular to c)
Alkali FeldsparHand specimenPink colour most characteristicTranslucent/opaqueTwo sets of cleavagePearly lustreMonoclinic (except microcline)Thin sectionLow birefringenceLow reliefCleavage (two sets in sectionsPerpendicular to c)
Microcline
Light Minerals: Quartz and Feldspars
Most acid igneous rocks are dominated by quartz and feldspar as the main minerals.Usually in granitic rocks there are two feldspars, one alkali and one plagioclase (albite).
Alkali FeldsparHand specimenPink colour most characteristicTranslucent/opaqueTwo sets of cleavagePearly lustreMonoclinic (except microcline)Thin sectionLow birefringenceLow reliefCleavage (two sets in sectionsPerpendicular to c)
Plag FeldsparHand specimenGreenish white most characteristicTranslucent/opaqueTwo sets of cleavagePearly lustreTriclinicThin sectionLow birefringenceLow reliefCleavage (two sets in sectionsPerpendicular to c)Polysynthetic Twinning
Light Minerals: Quartz and Feldspars
Most acid igneous rocks are dominated by quartz and feldspar as the main minerals.Usually in granitic rocks there are two feldspars, one alkali and one plagioclase (albite).
Alkali FeldsparHand specimenPink colour most characteristicTranslucent/opaqueTwo sets of cleavagePearly lustreMonoclinic (except microcline)Thin sectionLow birefringenceLow reliefCleavage (two sets in sectionsPerpendicular to c)
Plag FeldsparHand specimenGreenish white most characteristicTranslucent/opaqueTwo sets of cleavagePearly lustreTriclinicThin sectionLow birefringenceLow reliefCleavage (two sets in sectionsPerpendicular to c)Polysynthetic Twinning
Albite
Light Minerals: Some Problems with Feldspars
In many granitic (i.e coarse-grained acid rocks) alkali and plagioclase feldspars are mixed together as perthite (an exsolution texture).
Plagioclase lamellae within orthoclase (K-feldspar) Plagioclase lamellae within microcline (K-feldspar)
Light Minerals: Some Problems with Feldspars
In many granitic (i.e coarse-grained acid rocks) alkali and plagioclase feldspars are mixed together as perthite (an exsolution texture).
Light Minerals: Muscovite
MuscoviteOne excellent cleavageTransparent (in small flakes)Appear “silvery/golden appearance”
Light Minerals: Muscovite
High birefringence in XPLClear in PPL with low reliefUsually has a pitted appearance
Dark Minerals: Biotite
Colour: Black to brownCleavage: One excellent, forms books.
Brown, yellowish brown,Greenish brown in PPLStrongly Pleochroic
Dark Minerals: Hornblende
Yellow-green, brown-green, blue-green, brown in PPLStrongly pleochroic.124 degree cleavageCan look like biotite in thin-section but has higher relief.
Rock Types
Acid Igneous Rocks > 66 wt% silica
Rhyolite: >20% Quartz; 10-35% of the feldspar is plagioclase.
Rock Types
• Most rhyolites are mostly glassy and/or banded.
• Bands are devitrified zones.
• A devitrified rhyolite is often called a pitchstone.
• A completely glassy black rhyolite consists of obsidian.
Rock Types: Field names
Some useful rock names.
(1) Porphyry – Any leucocratic intrusive rock with phenocrysts in a fine-grained groundmass. Usually named after the most common phenocryst (e.g. “Quartz Porphyry”).
(2) Aplite – An equigranular leucocratic intrusive rock.(3) Felsite – An equigranular leucocratic rock dominated by
feldspar.(4) Pegmatite – A leucocratic rock where most crystals are >3cm
in size.
The Practical
20% of coursework
• Name all six specimens.• Give a short description of the 5 hand specimens including
mineralogy and texture. No sketch is required.• Give a full description with schematic sketch of the thin-
section of the “muscovite granite”.
• Hand-in date: End of next practical session.