Crystallochemistry and surface properties of biogenic opal

Abstract

SUMMARY The very low solubility of phytoliths or biogenic silicas (which are minor constituents of most soils) and their ability to stabilize nitrogenous fulvic acids in podzols are attributed to chemisorbed Al VI on the surface making the silica less soluble and the surface OH groups very reactive towards organic anions in an acid medium. The surface Si/Al ratio of phytoliths is very low (1.5–8) in comparison with the Si/Al ratio (8–20) for the whole sample. The pK values of the reactions in which surface groups are involved are pK 1 = 1.8 for deciduous phytoliths to 3.8 for more aluminous coniferous phytoliths; the pK 2 range is 8.2–9.0. The zero point of charge which is correlated with pK values varies from 5 to 6.