This study aims to understand the alteration of the primary minerals, epidote (sorosilicate) and microcline (tectosilicate) by humic acids (HAs) of diverse origin (extracted from Entisol, Alfisol and Mollisol). Kinetics of dissolution as well as nature of residues formed after weathering, were investigated. The solubilisation process represents a cycle of alternate dissolution and precipitation as seen from the recurring crests and troughs in the kinetic curves. The release of cations from the minerals is determined by its position in the crystal lattice, its strength of chelation with HA, accessibility of HA to the ion and the how the neighbouring ions are affected by its removal. Analysis of the residual products further illustrates the contrasting stabilities of the minerals. The composition of epidote showed significant reduction in Si4+, Fe2+/3+ and Ca2+ and enrichment in Al3+ with all the HAs; microcline showed only slight reduction in Si4+, Fe2+/3+ and K+ and enrichment in Al3+. The major XRD bands of microcline were unchanged in the residues and no new phases were observed. Epidote residue showed significant changes in many XRD bands and the formation of new phases. The IR spectra of both the minerals showed absorptions due to HA indicating the precipitation of metal-HA complex on mineral surface.
Weathering, silicates, epidote, microcline, humic acids, cation, solubilisation
