Experimental study of kyanite dissolution rates as a function of chemical affinity and solution composition

Kyanite steady state dissolution rates were measured in mixed flow reactors as a function of aqueous aluminum and silica concentration at pH 1.6 to 2.2 and at temperatures from 108 to 194 degrees C. Aqueous silica and aluminum concentration in these experiments ranged from 2.8 x 10(-7) less than or equal to [Si-total] less than or equal to 1.9 x 10(-4) mol/kg, and 3.0 x 10(-7) less than or equal to [Al-total] less than or equal to 4.1 x 10(-5) mol/kg, respectively, corresponding to chemical affinities ranging from similar to 40 to similar to 97 kJ/mol, Steady state dissolution was stoichiometric within experimental uncertainty. Dissolution rates were found to vary linearly with a(Al+3)(-1/2) and a(H+)(3/2) where a(i) designates the activity of the subscripted aqueous species. This behavior is consistent with the rates being controlled by the destruction of a neutrally charged aluminum deficient precursor complex at the mineral surface. Because of its dependence on aqueous aluminum, kyanite's constant temperature/pH steady state dissolution rates appear to depend on the chemical affinity of the overall dissolution reaction at these far from equilibrium conditions. All measured rates, which were obtained at far from equilibrium conditions where the contribution of reverse reaction is negligible, are consistent with r = A(A) exp (-E-A/RT)((a(H+)(3))/(a(Al+3)))(0.5), where r signifies the overall kyanite steady state dissolution rate normalized to a 1 cm(2) surface, A(A) refers to a pre-exponential factor equal to 2 x 10(-5) mol/cm(2)/sec, E-A designates an apparent activation energy equal to 75 kJ/mol, R represents the gas constant, and T denotes absolute temperature. Copyright (C) 1999 Elsevier Science Ltd.