Name

Composition

Crystal System

Optics

Occurrences

Beryl

Be₃Al₂Si₆O₁₈

Hexagonal

Uniaxial negative, low relief

Pegmatite mineral

(image 1)

(image 2)

(Na,Ca)(Li,Fe,Mg,Al)₃Al₆ (BO₃)₃Si₆O₁₈(OH)₄

Hexagonal/ rhombohedral

Uniaxial negative moderate relief

Pegmatite mineral: also occurs in some granites and boron-rich metamorphic rocks

Wollastonite

CaSiO₃

Triclinic

Biaxial negative moderate relief

Calc-silicate metamorphic rocks

Spodumene

LiAlSi₂O₆

Monoclinic

Biaxial positive moderate relief

Li-rich pegmatites and granites

Enstatite

Mg₂Si₂O₆

Orthorhombic

Biaxial positive moderate relief

Ultramafic rocks

Hypersthene

MgFeSi₂O₆

Orthorhombic

Biaxial positive, high relief

Mafic and ultramafic rocks; granulite facies metamorphic rocks

Diopside

CaMgSi₂O₆

Monoclinic

Biaxial positive high relief

Ultramafic rocks; calc-silicate metamorphic rocks

Augite: (image 1)(image 2)(image 3) (image 4)

(Ca,Fe,Mg,Na)(Mg,Fe, Al)Si₂O₆

Monoclinic

Biaxial positive high relief: moderately pleochroic

Common mineral in mafic igneous rocks

Aegirine (acmite)

NaFeSi₂O₆

Monoclinic

Biaxial negative very high relief; pleochroic green/brown

Alkaline igneous rocks