Assessing the solubilities and reaction kinetics of aluminous minerals in soils

Abstract

The use of chemical thermodynamics and reaction kinetics is necessary to quantitatively model the transformation of aluminous minerals and their dissolved constituents in soils and other geochemical systems. Soils are thermodynamically open systems subject to atmospheric and biological forces and do not attain overall thermodynamic equilibrium with respect to either mass or time. However, local or partial equilibrium conditions may persist for particular minerals and their dissolved constituents. Igneous and metamorphic primary minerals break down chemically to yield disordered gels or colloids and constituent ions, which can then reorganize or precipitate to form more stable hydrous oxides, silicates, carbonates or other mineral species. Naturally-occurring iron and aluminum hydrous oxides and kaolin clays, abundant in highly weathered soils, are commonly believed to be the ultimate, stable end products of weathering reactions, but usually are thermodynamically metastable with respect to more perfectly ordered, synthetic specimens. Thermodynamic stability is no guaranty of mineral persistence; with sufficient time, even the most perfectly crystallized, stable mineral will yield to the solubilizing assault of undersaturated surface waters. All of the dissolution and precipitation reactions of soil minerals are driven by energy differences in the thermodynamic stabilities of reactants and products, and the velocities (or kinetics) of such reactions are regulated by variables of the hydrogeochemical environment.