Weathering of Silicate Minerals by Humic Acids: I. Nature of Cation Solubilisation from Olivine and Tourmaline and Characteristics of the Residual Products

An attempt has been made here to understand the effect of HAs (extracted from Entisol, Alfisol and Mollisol) on the differential release of cations from the silicates, viz., olivine (neso) and tourmaline (cyclo), their relative order of stabilities, the kinetics of solubilisation processes and nature of the residual products on weathering. The kinetics curves of dissolution of both the minerals are unique in consisting of a series of recurring peaks and troughs signifying a cycle of alternate dissolution and reprecipitation. The release of cations from the structure being dependent on its respective position in the crystal lattice which in turn determines the access of HA to it; in addition, the stability of the chelate formed between HA and that particular cation is a determinant. Study of the residues of mineral-HA reaction showed that not only the easily weatherable olivine but also the resistant tourmaline has undergone significant changes including reduction in Fe^2+/3+ and Si⁴⁺ and enrichment in Al³⁺ and Mg²⁺ (olivine only). Emergence of new crystalline phases are evident from the XRD data while IR studies point out the deposition of HA on mineral surface probably after complexation with ionic constituents in the minerals.