Using information from the section -Composition of Common Igneous Rocks to ident

Question

Using information from the section -Composition of Common Igneous Rocks to identify the minerals in Fig. A54.1 Below each picture, write the name of the mineral, its chemical composition (chemical formula or chemical name), and t would allow you to recognize it in the future. All samples are shown at approximately their actual size. augite (pyroxene) biotite mica olivine hornblende (amphibole potassium-rich akali feldspar muscovite mica plagioclase feldspar quartz 2 4 6 8 gure A5.4.1 REFLECT& DISCUSS Which specific minerals are mafic and which ones are felsic? How do you know?

Explanation / Answer

1. Olivine - (Mg2+, Fe2+)2SiO4. Magnesium Iron Silicate. Olive green colour, Vitreous Lustre, Conchoidal to brittle fracture, Massive/Granular habit, Poor cleavage, Transparent to Translucent Diaphenity, Low specific Gravity, Biaxial positive.

2. Muscovite Mica- KAl2(AlSi3O10) (F,OH)2 . Potash Mica/ Isinglass/ Common Mica/ Hydrated phyllosilicate of potassium and magnesium. White, grey, silvery colour, Massiv to platy habit, Perfect cleavage, Micaceous fracture, SIlky/Pearly lustre, white streak, transparent to translucent diaphenity, Biaxial negative.

3. Plagioclase Feldspar- NaAlSi3O8 to CaAl2Si2O8. Albite to Anorthite. Grey Colour, Massive crystal habit, Tenacious, Perfect Cleavage, Twinning, Vitreous lustre, White streak, Transparent to translucent diaphenity

4. Biotite Mica - K(MgFe3AlSi3O10)(F,OH)2 Annite to Phlogopite. Dark brown colour, Platy habit, brittle to flexible, Vitreous lustre, White streak.

5. Hornblende- Ca2(Mg,Fe,Al)5(Al,Si)2O22(OH)2 . Black, Dark green colour, Hexagonal/Granular habit, Imperfect cleavage, uneven fracture, vitreous to dull lustre, Pale gray or gray-white streak.

6. Quartz- SiO2. Drusy, monocrystalline, 6-sided prism ending in 6-sided pyramid, Various colours, indistinct cleavage, brittle, conchoidal fracture, white streak, uniaxial positive, transparent,

7. Potassium-rich alkali feldspar- (Na,K)AlSi3O8. Gray, 2-3 cleavges, fracture along cleavage planes, vitreous lustre, white streak, opaque.

8. Augite (Pyroxene)- (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6. Black, brown, greenish colour, prismtic crystal habits, conchiodal fracture, good cleavage.

Mafic minerals are those which contains higher percentage of heavy minerals. Mafic minerals- Olivine, Pyroxene, Hornblende, Biotite Mica, Plagioclase Feldspar,

Felsic minerals are those which contains lower percentage of heavy minerals. Felsic minerals- Quartz, Muscovite Mica, Alkali Feldspar.

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