Practical 1 – Acid Igneous Rocks

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

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

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.