Abstract
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), 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. 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.
The principal structural difference among the three aluminum hydroxide polymorphs is in the mode of stacking successive layers. This, in turn, is controlled by the shape of, and charge distribution on, hydroxyl ions located at the surfaces of the layers. The extent of polarization by the aluminum cations determines whether the hydroxyl ions will possess cylindrical symmetry as in bayerite or tetrahedral symmetry as in gibbsite. We suggest that the extent of polarization of the hydroxyl ions within the solid phases is inherited from the polarization attained by the hydroxyl ions in aqueous aluminum complexes in the mother liquors.
The usual abundance of the silicate ion in natural alkaline environments may explain the scarcity of nordstrandite, for the silicate ion may favor the precipitation of alumino-silicate minerals rather than aluminum hydroxide. Near absence of bayerite in nature may indicate that it is metastable and will invert to nordstrandite in alkaline solutions.
