GEO 421 Petrology

Mineral Formulas

Igneous

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Tectosilicates Si:O = 1:2

Quartz SiO₂

Albite NaAlSi₃O₈ Anorthite CaAl₂Si₂O₈

Plagioclase (general formula): (Ca, Na)AlSI₃O₈

K-feldspar [sanidine, orthoclase, microcline] KAlSi₃O₈

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Inosilicates Si:O = 1:3

Orthopyroxene:

Hypersthene: (Mg, Fe)SiO₃

Enstatite MgSiO₃ Ferrosilite FeSiO₃

Clinopyroxene:

Diopside CaMgSi₂O₆ Hedenbergite CaFeSi₂O₆

Augite (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)₂O₆

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Nesosilicates (Orthosilicates) Si:O = 1:4

Olivine (general formula) (Mg,Fe)₂SiO₄

Forsterite Mg₂SiO₄ Fayalite Fe₂SiO₄

Important trends in mineral compositions:

WATER-

· ALL phyllosilicates are HYDROUS

That includes minerals or mineral groups like:

- mica (hydrated potassium-aluminum silicates)

- serpentine & talc (hydrated magnesium silicates)

- clay (hydrated aluminum silicates)

· ALL amphiboles are HYDROUS

ALKALIS (K & Na)-

· All feldspars & feldspathoids (except pure anorthite) are rich in, or contain alkalis

· All micas are rich in K or Li (lepidolite)

· Some clays are alkali-rich (e.g., ilite)

· Amphiboles and pyroxenes generally contain minor alkalis

CALCIUM-

· Ca-rich plagioclase feldspar

· ALL clinopyroxenes, e.g. augite and diopside, are Ca-rich

· ALL clinoamphiboles, e.g. hornblende, actinolite, tremolite, are Ca-rich

· Calcite (a minor igneous mineral) is obviously Ca-rich

ALUMINUM-

· ALL micas and clays are Al-rich

· ALL feldspars and feldspathoids are Al-rich

· Some amphiboles and, rarely, pyroxenes contain significant Al

IRON & MAGNESIUM-

· Most dark colored minerals contain iron to some degree; magnesium will commonly also be present