Cordierite

Formula:  Mg₂Al₄Si₅O₁₈

Origin of name:  For Pierre Cordier, French geologist

Crystal system:  Orthorhombic

Crystal class:  2/m 2/m 2/m

Twinning:  Common on {110} or {310}, simple, lamellar, or cyclic

Unit cell: a = 9.739 Å, b = 17.08 Å, c = 9.345 Å

Colour:  Blue, bluish violet, greenish, gray, yellowish, brown

Diaphaneity:

Luster:  Vitreous

Habit:  Crystals short prismatic; massive, compact, embedded grains

Hardness:  7 to 7.5

Specific gravity:  2.53 to 2.78

Cleavage:  3; {010} distinct, {001} and {100} indistinct

Tenacity:  Conchoidal; brittle

Streak:  Colourless

Synonyms/varieties:  Agalmatolite (massive alteration), iolite (gem var.), pinite (alteration), water-sapphire

Comments:  Series with sekaninaite; dimorphous with indialite; may be strongly pleochroic