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Formula: Al(OH)3
Hydroxide, paramorph of gibbsite,
nordstrandite and doyleite
Crystal System: Monoclinic
Specific gravity: 2.53 measured, 2.54 calculated
Streak: White
Colour: Colourless
Environments
Bayerite occurs in bauxite and as weathered crusts on
amphiboles and pyroxenes
(Webmin).
Localities
At the type locality, the Hatrurim Formation, Middle East, investigators used X-ray diffraction to identify the
bayerite, which occurs with calcite and
gypsum in veinlets cutting sedimentary rocks of late Cretaceous age. The rocks
are composed of calcite and spurrite.
The veinlets also contain vaterite,
portlandite, tobermorite group
minerals, thaumasite and
ettringite. Several of these minerals connote genesis at a very high pH
(strongly alkaline), and this agrees with the conditions under which pure bayerite is formed synthetically (pH>12).
(AM 55.43, Mindat). It is probably precipitated from high-alumina gels on carbonates at pH>5.8
(HOM).
At the Listvennoye deposit, Enisei Range, Krasnoyarsk Territory, Eastern-Siberian Region, Russia, bayerite occurs
as weathered crusts on amphiboles and
pyroxenes (HOM). Associated minerals include
gibbsite, diaspore and possibly
nordstrandite
(AM 55.43).
Alteration
Synthesis experiments in the alumina-water system at room temperature indicate that the
gibbsite polymorph precipitates slowly from solutions whose pH is below the
point of minimum solubility (pH 5.8), ie acid, and the bayerite polymorph precipitates rapidly from solutions
whose pH is above 5.8. Nordstrandite, the third
polymorph of aluminum
hydroxide, forms from bayerite during aging at intermediate to high pH values (alkaline to very alkaline). 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's of the mother liquors decrease if gibbsite precipitates and
increase if bayerite precipitates
(AM 55.43).
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