WEATHERING OF SILICATE MINERALS BY ORGANIC-ACIDS .1. NATURE OF CATION SOLUBILIZATION

The solubilisation of olivine, epidote, hornblende, tourmaline, biotite and microcline by oxalic, citric, salicylic acids and glycine were studied. The results indicate that organic acids dissolve minerals by a combined action of complexation and acid attack. Relative solubilities of the cations from a mineral are greatly dependent on their positions within the crystal and the crystal structure as a whole. These factors determine the accessibility of the ligand ions to the cations and also the extent to which the removal of one ion affects the neighbouring ions. Ions in polymeric chains or in sterically hindered positions are most resistant to dissolution. There is no unique stability sequence of the silicate minerals in different acids. The sequences are determined by the combined effect of two factors, viz., the crystal structure and the forces of interaction of the constituent ions with the surrounding medium. Kinetic studies indicate that the rate determining step at the initial stages of reaction is surface detachment of cation-ligand complexes; subsequently, the creation of fresh surfaces by fragmentation, etching. etc., may become rate-limiting.