THE FORMATION OF HYDROXYALUMINOSILICATES OF GEOCHEMICAL AND BIOLOGICAL SIGNIFICANCE

Abstract

Hydroxyaluminosilicates (HAS) are critical intermediates in the biogeochemical cycles of alumi-nium and silicon. To understand the extent of their role in controlling the solubility of Al in soil and surface waters, we need to know how they are formed and why they are formed in preference to other more soluble aluminous mineral phases. We have used a number of complementary analytical techniques to demonstrate the hitherto unproven mechanism of HAS formation of varying structure and stoichiometry. HAS were formed via the competitive condensation of silicic acid, Si(OH) 4 , at a hydroxyaluminum template (HAS A). Where Si(OH) 4 was present in excess, HAS A acted as a template for further reactions with Si(OH) 4 (HAS B). HAS A and HAS B had idealised Si : Al ratios of 0.5 and 1.0, respectively, and were representative of HAS found in soil horizons. This mechanism of formation of HAS could be used to both explain and predict the role of Si(OH) 4 in Al solubility control in the natural environment.