Name

Composition

Crystal System

Optics

Occurrences

Hornblende

(Na,K)(Ca,Na,Fe,Mg)₂(Mg,Fe,Al)₅ (Si,Al)₈O₂₂(OH)₂

Monoclinic

Biaxial negative; strongly pleochroic

Common mineral in mafic to intermediate igneous and metamorphic rocks

Tremolite

Ca₂Mg₅Si₈O₂₂(OH)₂

Monoclinic

Biaxial negative

Found in Mg-rich metamorphic rocks (ultramafics and calc-silicates)

Actinolite

Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂

Monoclinic

Biaxial negative; pleochroic green

Common in Fe,Mg - rich metamorphic rocks.

Edenite

NaCa₂(Mg,Fe)₅Si₇AlO₂₂(OH)₂

Monoclinic

Biaxial negative: often green in TS

Occurs in mafic/ultramafic metamorphic rocks - grades into hornblende

Riebeckite

Na₂Fe₅Si₈O₂₂(OH)₂

Monoclinic

Biaxial positive or negative; Pleochroic; blue in TS!!

Alkaline rocks

Glaucophane

Na₂Mg₃Al₂(Si₈O₂₂)(OH)₂

Monoclinic

Biaxial negative: Pleochroic purple in TS

Common mineral in subduction-complex metamorphic associations (high P, low T metamorphism)

Anthophyllite

(Mg,Fe)₇Si₈O₂₂(OH)₂

Orthorhombic

Biaxial negative

Amphibolite facies metamorphic mineral in ultramafic rocks: often asbestiform. Assoc. with olivine