Mineral

Composition

Crystal System

Optics

Occurrence/Other

Mica Group

Muscovite

KAl₃Si₃O₁₀(OH)₂

Monoclinic

Biaxial negative.

Common in felsic igneous rocks, and pelitic metamorphic rocks; also an alteration phase (var.: sericite)

Biotite

K(Mg,Fe)₃AlSi₃O₁₀(OH)₂

Monoclinic

Biaxial negative; strongly pleochroic; brown-red colors

Common in Fe-rich igneous and metamorphic rocks: found with muscovite, feldspars, garnets

Phlogopite

KMg₃AlSi₃O₁₀(OH)₂

Monoclinic

Biaxial negative: may be pleochroic

Found in ultamafic rocks as a primary phase, or due to interactions with Si-rich fluids/melts: alters to vermiculite

Lepidolite

K(Li,Al)_2-3AlSi₃O₁₀(OH)₂

Monoclinic

Biaxial negative

Distinctive lavender color; found in Li-rich granites and pegmatites

Chlorite

(Mg,Fe,Al)₆(Si,Al)₄O₁₀(OH)₈

Monoclinic

Biaxial negative; Pleochroic green; low birefringence colors

Ubiquitous mineral in low to mid-grade metamorphic rocks. Large solid solution variability.

Margarite

KAl₄Si₂O₁₀(OH)₂

Monoclinic

Biaxial negative

Forms in Al-rich metamorphic rocks; pale green and less micaceous in habit.

Kaolinite

Al₄Si₄O₁₀(OH)₈

Triclinic

sorta irrelevant

white clay.

Prehnite

Ca₂Al(AlSi₃O₁₀)(OH)₂

Orthorhombic

Biaxial positive

Low temperature alteration of basalts; pale green; botryoidal habit.

Apophyllite

KCa₄Si₈O₂₀F•8H₂O

Tetragonal

Uniaxial positive

Occurs with prehnite and zeolites: low temperature basalt alteration

Serpentine

Mg₆Si₄O₁₀(OH)₈

Monoclinic

Biaxial negative: very low birefringence colors

Most common alteration mineral of olivine-rich rocks - occurs with anthophyllite, chlorite, talc, and other Mg-rich phases.

Varieties:

Garnierite is a Ni-rich variety.

Talc

Mg₃Si₄O₁₀(OH)₂

Monoclinic

Biaxial negative: smeared in TS

Common alteration mineral of ultramafic rocks - Soapstone is the rock type.

Pyrophyllite

Al₂Si₄O₁₀(OH)₂

Triclinic

Biaxial negative: high birefringence

Typically forms rosette-like crystals: similar to talc. Low and medium grade metamorphic rocks.