EXPERIMENTAL AND MATHEMATICAL MODELING OF THE KINETICS OF CONGRUENT AND INCONGRUENT DISSOLUTION REACTIONS OF ALKALINE FELDSPARS

Abstract

Concentrations of dissolved elements were determined experimentally for the reactions of sanidine congruent dissolution. in 0.1 m KHCO 3 solution, albite transformation into analcime in 0.1 m solution of NaHCO3, and albite transformation into sanidine in 0.1 m solution of KHCO3 at 300°C and pH 9. The minerals were preliminarily repeatedly treated by fresh solution portions to remove dust particles and surface defects produced during sample crushing and grinding. Sanidine dissolution was associated with the development of a leached surface layer depleted in Al. The layer attained a maximal thickness of 0.1 μm at equilibrium and then disappeared. In the reaction of albite transformation into sanidine, the Si/Al ratio in the solution exceeded the stoichiometric ratio by 100% because of insignificant (1%) deviations from this ratio in the dissolving albite or precipitating sanidine. The experimental data on all reactions were approximated by the equation r = -kS[1 - (Q/K)1/σ], where r is the reaction rate, k is the reaction rate constant, S is the surface area of the mineral (it was calculated from the grain sizes), Q and K are the quotient and equilibrium constant of the reaction, and σ is a coefficient. The values of k (μmol/m 2 s), σ and K were determined for the reactions of sanidine dissolution (0.55, 3.3, and 1.13 × 10-13), sanidine precipitation (0.07, 1.2, and 1.62 × 10-13), and albite dissolution in the reactions of albite transformation into analcime (0.26, 1, and 8.03 × 10-13) and sanidine (0.07, 1.2, and 6.65 × 10-13). The principle of microscopic-scale reversibility was not fulfilled for the reactions of albite and sanidine dissolution and precipitation but was valid for the reactions of albite dissolution and sanidine precipitation, and, thus, the active surface areas at which both reactions proceeded were concluded to be equivalent. The drastic deceleration of albite dissolution early in the incongruent stage is explained by the blocking of the active dissolution areas with precipitating sanidine. The reactions of sanidine congruent dissolution and albite transformations into analcime and sanidine were simulated mathematically by solving systems of differential equations in which the variations in the concentrations of dissolved elements with time were described through the dissolution rates of the primary mineral and the precipitation rate of the secondary mineral with regard for the reaction stoichiometry. This kinetic model is in good agreement with experimental data and can be applied to other mineral transformation reactions.