Mineral

TS?

Composition

Crystal System

Optics

Properties/Occurrence

Andalusite

Yes

Al₂SiO₅

Orthorhombic

Biaxial negative; moderately high relief

Low grade metamorphic mineral - variety Chiastolite shows a "Maltese cross" when cut in prism cross-section.

Kyanite

No*

Al₂SiO₅

Triclinic

Biaxial negative; high positive relief

Typically blue and bladed - found in high pressure metamorphic rocks - differential hardness

Sillimanite

Yes

Al₂SiO₅

Orthorhombic

Biaxial positive; moderately high relief

Found in high Temperature metamorphic rocks - forms fibrous white crystals, which can occur in "felty" aggregates

Olivine

Yes

(Mg,Fe)₂SiO₄

Orthorhombic

Biaxial positive for Mg-rich varieties (Forsterite); negative for Fe-rich varieties (Fayalite)

Forsteritic olivine is a primary mantle mineral - found with enstatite, diopside, chromite, and Ca-rich plagioclase feldspars. Alters readily to serpentine. Fayalitic olivine can be found in Fe-rich granitic rocks, or in Fe-rich metamorphic rocks.

Garnet

Yes

Mg₃Al₂(SiO₄)₃ (Pyrope); Fe₃Al₂(SiO₄)₃ (almandine); Mn₃Al₂(SiO₄)₃ (spessartine); Ca₃Al₂(SiO₄)₃ (Grossular); Ca₃Fe₂(SiO₄)₃ (Andradite); Ca₃Cr₂(SiO₄)₃ (uvarovite): generic formula: A⁺²₃B⁺³₂(SiO₄)₃

Isometric

Isotropic

Very common mineral in mid- to high grade metamorphic rocks of all compositions: also occurs in some ultramafic igneous rocks. Colors can vary markedly (Uvarovite: emerald green; Melanite: black; Pyrope: deep red; Grossular: pale green to brown.

Topaz

Yes

Al₂SiO₄(F,OH)₂

Orthorhombic

Biaxial positive; moderate relief

Found as a minor constituent in Si-rich igneous rocks.

Epidote

Yes

Ca₂(Al,Fe)Al₂Si₃O₁₂(OH)

Monoclinic

Biaxial negative; high relief, sometimes slightly pleochroic; may be pale green or pinkish in thin section

Common low to mid-grade metamorphic mineral

Staurolite

Yes

Fe₂Al₉Si₄O₂₃(OH)

Monoclinic

Biaxial positive; high relief, often pleochroic - yellow to brown in thin section.

Common in Fe-rich metamorphic rocks - a diagnostic metamorphic mineral.