Gibbsite

hydroxide

kaolinite

Formula:Al(OH)3 hydroxide
Specific gravity:
Hardness: 2½ to 3
Streak: White
Colour: White, light gray, light green, reddish white; reddish yellow (impure)
Solubility: Insoluble in water, hydrochloricand nitric acid; soluble in sulphuric acid

Plutonic igneous environments
Pegmatites
Sedimentary environments

Gibbsite is a secondary mineral forming in the weathered surface zones in clay deposits and limestone, as well as low-silica igneous rocks and pegmatites. It is a constituent of the aluminium ore bauxite.

Alteration

kaolinite and H2O to gibbsite and quartz
Al2Si2O5(OH)4 + H2O ⇌ 2Al(OH)3 + 2SiO2

Common impurities: Fe,Ga

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