CHANGE IN THE DISSOLUTION RATES OF ALKALI FELDSPARS AS A RESULT OF SECONDARY MINERAL PRECIPITATION AND APPROACH TO EQUILIBRIUM

Reaction rates of congruent and incongruent dissolution of sanidine in NaHCO3 solution (0.1 m) and albite in KHCO3 solution (0.1 m) as well as the reaction rates of precipitation of secondary minerals (analcime and sanidine) have been measured at 300 degrees C, 88 bars, and pH 9. At congruent stage, the reaction rates change, as equilibrium is appr oached, according to the following law: r = +/-kS\1 - (Q/K)(p)\(q) whe re k is the rate constant, S is the surface area of the mineral, Q is the activity quotient, K is the equilibrium constant, and p and q are fitted parameters. For dissolution of sanidine and albite, the values of k (mol/m(2)/sec), p, q are equal to 5.0 . 10(-7), 0.16, 1.4, and 4. 5 . 10(-7), 0.76, 90, respectively. Strong correlation between p and q has been revealed allowing us to reduce the number of fitted paramete rs. Deviation of p and q from unity suggests a complex dissolution mec hanism involving crystal defects and/or a number of parallel and succe ssive elementary reactions with comparable rates. Incongruent dissolut ion of the primary minerals is controlled by precipitation reactions o f secondary ones. Precipitation rates may also obey the rate law given above, but determination of coefficients in this case was difficult d ue to change of S in processes of nucleation, growth, coalescence of c rystals, and formation of pores filled with primary minerals. Correlat ion between reaction rates of primary mineral dissolution and secondar y mineral precipitation is more rigid than one might expect from simul taneous solution of two rate equations. Under conditions of low kineti c activity of the surface, this rigid correlation can be explained, by analogy with electrode processes, using a source-sink mechanism. The experimental data obtained for the most widespread and typical rock-fo rming minerals are of major importance to improving kinetic models of water-rock interaction. Copyright (C) 1997 Elsevier Science Ltd.