An important deficiency in the thermodynamic data for common rock-forming minerals involves chamosite, the Fe2+ end-member of chlorite solid solution. The ΔGf0 and ΔHf0 values for chamosite reported in the literature, as computed from both phase-equilibria data and empirical approaches, exhibit huge discrepancies of up to 210 kJ/mol. A small portion of the discrepancy (5–10%) is related to inconsistencies in the Al reference state chosen for the thermodynamic retrieval calculations. It is shown that the remainder can be assigned to an improper application of the van't Hoff relationship to compute standard-state thermodynamic properties (25 °C, 1 bar) for chamosite from equilibrium constants derived from high-temperature (575–625 °C) and high-pressure (2.07–6.00 kbar) phase-equilibria experiments. We have reinterpreted these previously published experimental results by taking explicit account of heat-capacity and entropy data for chamosite predicted from additivity models. Following this approach, we compute ΔGf0 and ΔHf0 for chamosite of −6495.13 ± 4.17 and −7101.91 ± 4.17 kJ/mol, respectively, values that are in excellent agreement with those computed from empirical techniques, after inconsistencies related to the Al reference state are taken into account. In this way, major discrepancies in previously reported thermodynamic data for chamosite are resolved.

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