Morphology of Nordstrandite

P. Violante1, A. Violante1 and J. M. Tait2
1 Istituto di Chimica Agraria, Universita di Napoli, 80055 Portici, Italy.
2 The Macaulay Institute for Soil Research, Craigiebuckler, Aberdeen AB9 2QJ, Scotland, United Kingdom.

Abstract: Nordstrandite was obtained synthetically by precipitation of Al(OH)3 at pH 9.0 to 11.0 in citrate or malate systems with Al: carboxylic acid molar ratios of 10–100 in the absence and in the presence of montmorillonite. Examination by transmission and scanning electron microscopy showed that the nordstrandite had a tabular morphology in the clay-free systems with Al: carboxylic acid ratio >50. Because the inhibiting effect on crystallization of the citrate or malat e anion decreases with increase in pH, the crystal size varied from <2 µm at pH 10 to 5 µm at pH 11. With increasing concentration of citrate or malate, the nordstrandite crystals became more elongated in the c-direction probably due to organic anion occupation of the coordination sites of Al on the edge faces parallel to the c-axis. Irregular growth along the c-axis was probably due to steric hindrance by the organic ligands distorting the arrangement of the unit layers of nordstrandite.

The catalytic effect of montmorillonite on the formation of nordstrandite was confirmed. Nordstrandite synthesized in the presence of montmoriUonite at pH 9.0 presented an ill-defined ovoidal outline (0.2–0.5 µm), some showing shafts of bayerite growing from the center. At higher pH, where conditions for crystalline growth were again more favorable, nordstrandite crystals nucleated on the montmorillonite surfaces and condensed in clusters of weak face-to-face associations of plates.

Key Words: Carboxylic acid • Electron diffraction • Electron microscopy • Montmorillonite • Morphology • Nordstrandite • Synthesis

Clays and Clay Minerals; December 1982 v. 30; no. 6; p. 431-437; DOI: 10.1346/CCMN.1982.0300605
© 1982, The Clay Minerals Society
Clay Minerals Society (www.clays.org)