Abstract

The technique that we developed to estimate ΔGf0 of silicate minerals by summing the contribution of polyhedral components at 298 K (Chermak and Rimstidt, 1989) can be extended to predict the free energy of formation of silicate minerals at higher temperatures. This approach is particularly useful for clays and zeolites, which are geologically abundant and geochemically reactive but have few data for their ΔGf0 at temperatures above 25 °C. Multiple linear regression (MLR) was used to find the contribution of [4]Al2O3-[6]Al2O3-[6]Al(0H)3-[4]SiO2-[6]Mg0-[6]Mg(0H)2-[6]CaO-[8-z]CaO-[6-8]Na2O-[8-12]K2O-[6]FeO-[6]Fe(0H)2-[6]Fe2O3 components to the total ΔGf0 and ΔHf0 at 298 K for a selected group of silicate minerals (Chermak and Rimstidt, 1989). The gi coefficients from this model can be extrapolated to higher temperatures using the equation

gi,T=hi,298T(hi,298gi,298298),

where gi and hi are the respective molar free energy and enthalpy contribution of one mole of each oxide or hydroxide component at 298 K. The gi of H2O was found to vary depending upon the cation with which the H2O was associated. Therefore, gH2O was determined by two linear regressions: one for Na+ and the other for Ca2+. Experimentally measured ΔGf0 vs. predicted ΔGf0 for 21 minerals used in the model have associated differences of 0.19%, 0.20%, and 0.22% at 400, 500, and 600 K, respectively. Experimentally measured ΔGf0 vs. predicted ΔGf0 for four, three, and two minerals not used in the model have associated differences of 0.36%, 0.34%, and 0.13% at 400, 500, and 600 K, respectively.

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