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Formula:Al(OH)3
Hydroxide, paramorph of bayerite,
doyleite and nordstrandite
Crystal System: Monoclinic
Specific gravity: 2.38 to 2.42 measured, 2.44 calculated
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 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).
Gibbsite from the Adelaide Mine - Image
At the Xianghualing Sn-polymetallic ore field, Linwu County, Chenzhou, Hunan, China, pale turquoise-blue botryoidal
gibbsite occurs covering surfaces in voids in a dark grey matrix
(AESS).
Gibbsite from Xianghualing - Image
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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