Alumino-ferrotschermakite

Formula:  Ca₂(Fe,Mg)₃(Al,Fe)₂(Si₆Al₂)O₂₂(OH)₂

Origin of name:  From aluminum, iron (Latin ferrium) and tschermakite

Crystal system:  Monoclinic

Crystal class:  2/m

Twinning: Simple or multiple parallel {100}

Unit cell: a = 9.76 Å, b = 17.99 Å, c = 5.33 Å, b = 105.10°

Colour: Green

Diaphaneity: Transparent to translucent

Luster: Vitreous

Habit: prismatic crystals, as reaction rims on other minerals

Hardness: 5-6

Specific gravity: about 3.15

Cleavage: 3; {110} perfect, {100} and {001} parting

Tenacity: Brittle

Streak:

Synonyms/varieties:

Comments:  Series with ferrotschermakite and with tschermakite. Member of the amphibole group. Formed in eclogites and other ultramafic igneous rocks, and in medium- to high-grade metamorphic rocks, typically amphibolites.