Gibbsite

minerals

philipsbornite

diaspore

doyleite

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Formula:Al(OH)3
Hydroxide, paramorph of bayerite, doyleite and nordstrandite
Specific gravity:
Hardness: 2½ to 3
Streak: White
Colour: White, light gray, light green, reddish white; reddish yellow (impure)
Solubility: Insoluble in water, hydrochloric and nitric acid; soluble in sulphuric acid
Common impurities: Fe,Ga

Plutonic igneous environments
Pegmatites
Metamorphic environments
Hydrothermal 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 typical product of weathering of aluminous minerals, and it also forms in low-temperature hydrothermal and metamorphic environments, replacing aluminous minerals. It is an important constituent of bauxite, and may be the chief aluminium mineral present (Dana, HOM). Associated minerals include diaspore, böhmite, corundum, kaolinite and goethite (HOM).

Localities

At the Adelaide Mine, Dundas mineral field, Zeehan District, West Coast municipality, Tasmania, Australia, gibbsite is abundant and commonly overgrows crocoite. It may also be overgrown by crocoite and commonly forms moulds after dundasite (AJM 12.2.81).

At the type locality, Richmond, Berkshire county, Massachusetts, USA, gibbsite occurs in a bog iron deposit associated with limonite and goethite (Mindat).

Alteration

Synthesis experiments indicate that gibbsite precipitates slowly from solutions whose pH is below 5.8, and the paramorph bayerite precipitates rapidly from solutions whose pH is above 5.8. Nordstrandite, the third paramorph of aluminum hydroxide, forms from bayerite during aging at intermediate to high pH values. In solutions of intermediate pH, both gibbsite and bayerite form, but with aging, early-formed gibbsite disappears as more bayerite forms. During aging, the pH of the mother solution decreases if gibbsite precipitates and increases if bayerite precipitates (AM 55.43-77).

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

Philipsbornite, H+ and H2O to Pb2+, gibbsite and H3AsO4
PbAl3(AsO4)(AsO3OH)(OH)6(s) + 2H+(aq) +3H2O(l) = Pb2+(aq) + 3Al(OH)3(s) + 2H3AsO4(aq)
(LMW p269)

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