SYNTHETIC ALLOPHANES FORMED IN CALCAREOUS ENVIRONMENTS - NATURE, CONDITIONS OF FORMATION, AND TRANSFORMATIONS

Synthetic allophanes were prepared by adding excess CaCO3 to solutions containing Al(NO3)3 and Si(OH)4 (concentrations less-than-or-equal-to 1.5 mM) with molar Si/Al starting ratios ranging from 1:8 to 4:1. The systems were incubated at 20 and 80-degrees-C until the concentration of Si(OH)4 in solution approached stability after 6 to 10 wk, when the allophanes were isolated and characterized by infrared (IR) spectroscopy and electron microscopy. At 20-degrees-C, proto-imogolite allophane, rather than Al(OH)3 species, formed when Si in solution exceeded about 50-mu-M, but hydrous feldspathoids, containing tetrahedral Al, were progressively formed when Si in solution exceeded about 150-mu-M. At 80-degrees-C, partial or nearly complete transformation of these products to proto-halloysite occurred for Si in solution above 50-mu-M. The presence of 1 mM Mg and K in the starting solutions did not affect the allophanes formed at 20-degrees-C, but modified those formed at 80-degrees-C. For starting Si/Al ratios of 1:2 and 1:1, the formation of proto-halloysite was partially inhibited, but for starting ratios of 2:1 and 4:1, evidence for the initiation of 2:1 trioctahedral layer silicate saponite was obtained. It is postulated that an alkaline environment favors the transformation of allophanes to layer silicate clays, and that this may account for the scarcity of reports of allophanes in calcareous horizons. A marked decrease in Si in solution when acid soils are limed is explained.