Gibbsite

Formula:  Al(OH)₃

Origin of name:  For George Gibbs, American collector

Crystal system:  Monoclinic

Crystal class:  2/m

Twinning:  Common about [130] axis; common on {001}; rare on {110} or {100}

Unit cell: a = 8.66 Ǻ, b = 5.08 Ǻ, c = 9.70 Ǻ, β = 94.2°

Colour:  White, grayish, greenish, reddish white

Diaphaneity:

Luster:  Vitreous, pearly on cleavage

Habit:  Crystals tabular, hexagonal to large size; massive, chalcedony-like coatings and crusts; stalactitic, concretionary, fibrous

Hardness:  2.5 to 3.5

Specific gravity:  2.40

Cleavage:  1; {001} perfect

Tenacity:  Tough

Streak: 

Synonyms/varieties:

Comments:  Polymorphous with bayerite, doyleite and nordstrandite