Practical 1 – Acid Igneous Rocks

1. Practical 1 – Acid Igneous Rocks

2. 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 Feldspar Hand specimen Pink colour most characteristic Translucent/opaque Two sets of cleavage Pearly lustre Monoclinic (except microcline) Thin section Low birefringence Low relief Cleavage (two sets in sections Perpendicular to c)

3. 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 Feldspar Hand specimen Pink colour most characteristic Translucent/opaque Two sets of cleavage Pearly lustre Monoclinic (except microcline) Thin section Low birefringence Low relief Cleavage (two sets in sections Perpendicular to c) Quartz Hand specimen White, grey, yellowish white Translucent to transparent No cleavage but concoidal fracture Vitreous lustre Trigonal Thin section Low birefringence Low relief Lack of cleavage

4. 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 Feldspar Hand specimen Pink colour most characteristic Translucent/opaque Two sets of cleavage Pearly lustre Monoclinic (except microcline) Thin section Low birefringence Low relief Cleavage (two sets in sections Perpendicular to c) Alkali Feldspar Hand specimen Pink colour most characteristic Translucent/opaque Two sets of cleavage Pearly lustre Monoclinic (except microcline) Thin section Low birefringence Low relief Cleavage (two sets in sections Perpendicular to c)

5. 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 Feldspar Hand specimen Pink colour most characteristic Translucent/opaque Two sets of cleavage Pearly lustre Monoclinic (except microcline) Thin section Low birefringence Low relief Cleavage (two sets in sections Perpendicular to c) Alkali Feldspar Hand specimen Pink colour most characteristic Translucent/opaque Two sets of cleavage Pearly lustre Monoclinic (except microcline) Thin section Low birefringence Low relief Cleavage (two sets in sections Perpendicular to c) Microcline

6. 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 Feldspar Hand specimen Pink colour most characteristic Translucent/opaque Two sets of cleavage Pearly lustre Monoclinic (except microcline) Thin section Low birefringence Low relief Cleavage (two sets in sections Perpendicular to c) Plag Feldspar Hand specimen Greenish white most characteristic Translucent/opaque Two sets of cleavage Pearly lustre Triclinic Thin section Low birefringence Low relief Cleavage (two sets in sections Perpendicular to c) Polysynthetic Twinning

7. 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 Feldspar Hand specimen Pink colour most characteristic Translucent/opaque Two sets of cleavage Pearly lustre Monoclinic (except microcline) Thin section Low birefringence Low relief Cleavage (two sets in sections Perpendicular to c) Plag Feldspar Hand specimen Greenish white most characteristic Translucent/opaque Two sets of cleavage Pearly lustre Triclinic Thin section Low birefringence Low relief Cleavage (two sets in sections Perpendicular to c) Polysynthetic Twinning Albite

8. 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)

9. 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).

10. Light Minerals: Muscovite Muscovite One excellent cleavage Transparent (in small flakes) Appear “silvery/golden appearance”

11. Light Minerals: Muscovite High birefringence in XPL Clear in PPL with low relief Usually has a pitted appearance

12. Dark Minerals: Biotite Colour: Black to brown Cleavage: One excellent, forms books. Brown, yellowish brown, Greenish brown in PPL Strongly Pleochroic

13. Dark Minerals: Hornblende

14. Dark Minerals: Hornblende Yellow-green, brown-green, blue-green, brown in PPL Strongly pleochroic. 124 degree cleavage Can look like biotite in thin-section but has higher relief.

15. Rock Types Acid Igneous Rocks > 66 wt% silica

16. Rock Types Acid Igneous Rocks > 66 wt% silica Rhyolite: >20% Quartz; 10-35% of the feldspar is plagioclase.

17. 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.

18. Rock Types: Field names • Some useful rock names. • Porphyry – Any leucocratic intrusive rock with phenocrysts in a fine-grained groundmass. Usually named after the most common phenocryst (e.g. “Quartz Porphyry”). • Aplite – An equigranular leucocratic intrusive rock. • Felsite – An equigranular leucocratic rock dominated by feldspar. • Pegmatite – A leucocratic rock where most crystals are >3cm in size.

19. 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.