Hydroxide, paramorph of bayerite, doyleite and nordstrandite
Hardness: 2½ to 3
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
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
Associated minerals include diaspore, böhmite,
corundum, kaolinite and
At the Mount Kelly deposit, Gunpowder District, Queensland, Australia, the deposit has been mined for oxide and supergene copper ores, predominantly malachite, azurite and chrysocolla. The ores overlie primary zone mineralisation consisting of quartz-dolomite-sulphide veins hosted in dolomite-bearing siltstone and graphitic schist.
Gibbsite is rare and found primarily associated with kaolinite (AJM 22.1.17).
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).
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)
Back to Minerals