NOTE CO-CRYSTALL IZAT ION OF G IBBS ITE AND BAYERITE WITH

Gibbsite, nordstrandite and bayerite--the three polymorphs of aluminium hydroxide, Al(OH)3---occur naturally in soil clays and can be synthesized in the laboratory. Recently, some of the factors influencing the nature of synthetic aluminous products have been identified (Violante & Jackson, 1979, 1981; Violante & Violante, 1980). These reports have shown, for example, that at pH 9-11 and in the presence of carboxylic acids, bayerite crystallization is inhibited in favour of gibbsite and nordstrandite. The addition of montmoriUonite o the system further facilitates gibbsite and nordstrandite formation. Electron microscopy was used by Violante et al. (1982) to study the variation in crystal habit of nordstrandite synthesized under different conditions of pH and A1/carboxylic acid molar ratios with or without montmorillonite. In clay-free systems at pH 9-10 and Al/citric or malic acid molar ratios of 50-100, nordstrandite occurred as euhedral crystals displaying a characteristic tabular habit. With increasing concentration of citrate or malate, the nordstrandite crystals showed preferential growth in the crystallographic c direction. When synthesized in the presence of montmorillonite at pH 9, nordstrandite crystallized as ill-defined ovoidal particles, while at higher pH, radiating clusters of thi