The variation of quartz and aluminosilicates dissolution rates as a function of solution composition reveals two distinct types of behaviors (Oelkers and Schott, 1995). Minerals such as single (hydr)oxides (including quartz (Berger et al., 1994)) and some multi (hydr)oxides (e.g. anorthite (Oelkers and Schott, 1995)) have dissolution mechanisms that require the breaking of only one type of metal-oxygen bond. Consequently, the rate controlling precursor complexes for these minerals are formed by simple adsorption reactions, and their constant temperature/pH steady state dissolution rates are independent of chemical affinity at far from equilibrium conditions (A>>RT, where A refers to the chemical affinity of the dissolving mineral, R designates the gas constant, and T refers to absolute temperature) (Schott and Oelkers, 1995). In contrast, aluminosilicate minerals such as albite (Oelkers et al., 1994), K-feldspar (Gautier et al., 1994), kyanite (Oelkers and Schott, 1994), and kaolinite (Devidal, 1994) have dissolution mechanisms that require the breaking of two or more types of bonds. Consequently, the rate controlling precursor complexes for these minerals are formed by a combination of exchange and adsorption reactions. Owing to the stoichiometry of the hydrogen for aluminum exchange reaction forming the rate controlling precursor complexes for these minerals, their far from equilibrium dissolution rates are proportional to (a3H+/aAl+3)n, where ai designates the activity of the subscripted aqueous species. It has been demonstrated that the variation of these dissolution rates as a function of solution composition, including the effect of complexing agents like organic acids, can be quantified through their effect on the (a3H+/aAl+3) aqueous activity ratio (Oelkers and Schott, 1995).
To enable the generalization of these observations to other silicate mineral groups, steady-state dissolution rates of Bramble Enstatite (Mg0.86Fe0.14SiO3) were measured as a function of aqueous magnesium and silica concentration at acidic pH and temperatures ranging from 25 to 170°C in titanium mixed flow reactors. All rates were obtained at far from equilibrium conditions; rate data are consistent with an apparent activation energy of 40.6 kJ/mol. Resulting steady-state rates were found to be proportional to the activity ratio (a1/2H+/a1/4Mg+2), which is consistent with the concept that enstatite dissolution is a two step process: 1) the exchange of two aqueous hydrogen ions for one magnesium atom in the mineral structure forming a rate controlling precursor complex consisting of partially detached Si-tetrehedra, and 2) the irreversible detachment of this precursor complex. Because they depend on aqueous magnesium activity,
enstatite's constant temperature/pH steady state dissolution rates appear to depend on the chemical affinity at far from equilibrium conditions.
One major consequence of the differences in the constituents comprising the precursor forming reaction for enstatite versus the aluminosilicates albite, K-feldspar, kyanite, and kaolinite is the variation of their dissolution rates with pH. Because of the ready formation of the aqueous aluminum hydroxide complexes Al(OH)2+, Al(OH)2+, Al(OH)3, and Al(OH)4-, both the aqueous activity ratio (a3H+/aAl+3) and aluminosilicate dissolution rates exhibit a minimum at pH 4-6, depending on temperature. In contrast, aqueous magnesium-hydroxide complexes (Mg(OH)+, Mg(OH)2, and Mg(OH)3-) are relatively weak, and both (a1/2H+/a1/4Mg+2) and enstatite dissolution rates decrease monotonically with pH to at least pH ~ 10.
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