Abstract

Many physical properties of silicate minerals can be modeled as a combination of basic polyhedral units (Hazen, 1985, 1988). It follows that their thermodynamic properties could be modeled as the sum of polyhedral contributions. We have determined, by multiple regression, the contribution of the [4]A12O3,[6]A12O3, [6]Al(OH)3, [4]SiO2, [6]MgO, [6]Mg(OH)2,[6]CaO, [8-z]CaO, [6−8]Na2O, [8−12]K2O, H2O, [6]FeO, [6]Fe(OH)2, and [6]Fe2O3 components to the total ΔGf0and ΔHf0 of a selected group of silicate minerals. Using these data we can estimate the ΔGf0and ΔHf0 of other silicate minerals from a weighted sum of the contribution of each oxide and hydroxide component: ΔGf0=Σnigi,andΔHf0=Σnihi, where ni is the number of moles of the oxide or hydroxide per formula unit and gi and hi, are the respective molar free energy and enthalpy contribution of 1 mol of each oxide or hydroxide component. The technique outlined here can be used to estimate the thermodynamic properties of many silicate phases that are too complex or too impure to give reliable calorimetric measurements.

Experimentally measure ΔGf0and ΔHf0 vs. predicted ΔGf0and ΔHf0 for the minerals used in the model have associateda verager esiduals of 0.26% and 0.24% respectively. Thermodynamic properties of minerals not used in the model but for which there are experimentally determined calorimetric data have average differences between measured and predicted values of 0.25% for ΔGf0 or 18 minerals and 0.22% for ΔHf0 for 20 minerals.

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