Sillimanite

Formula: Al₂SiO₅

Origin of name: for B. Silliman, American chemist

Crystal system: orthorhombic

Crystal class: 2/m 2/m 2/m

Twinning:

Unit cell: a = 7.470 Å, b = 7.663 Å, c = 5.759 Å

Colour: colourless, white, gray, yellowish, brownish, greenish bluish

Diaphaneity: 

Luster: vitreous to silky

Habit: crystals long prismatic, vertically striated, nearly square cross section, massive, fibrous to somewhat columnar

Hardness: 6.5 to 7.5

Specific gravity: 3.23 to 3.27

Cleavage: 1; {010} perfect

Tenacity: uneven

Streak: colourless

Synonyms/varieties:

Comments: trimorphous with andalusite and kyanite