Riebeckite

Formula: Na₂(Fe,Mg)₃Fe₂Si₈O₂₂(OH)₂

Origin of name: for Emil Riebeck, German explorer

Crystal system: monoclnic

Crystal class: 2/m

Twinning: common on {100} simple or lamellar

Unit cell: 1 = 9.77 Å, b = 18.05 Å, c = 5.34 Å, b = 103.6°

Colour: dark blue to black

Diaphaneity: 

Luster: vitreous, silky

Habit: crystals long prismatic, striated along elongation, massive, fibrous to asbestiform, columnar, granular

Hardness: 5

Specific gravity: 3.32 to 3.382

Cleavage: 1; {110} perfect

Tenacity: uneven, brittle

Streak: 

Synonyms/varieties: crocidolite (fibrous variety), abriachanite, orthoriebeckite, osannite, mangancrocidolite (Mn-rich variety)

Comments: member of the amphibole group, series with magnesioriebeckite and arfvedsonite