A resolution of discrepant thermodynamic properties for chamosite retrieved from experimental and empirical techniques

An important deficiency in the thermodynamic data for common rock-forming minerals involves chamosite, the Fe²⁺ end-member of chlorite solid solution. The ΔG^o _f and δH^o _f 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. The remainder can be assigned to an improper application of the van't Hoff relationship to compute standard-state thermodynamic properties for chamosite from equilibrium constants derived from high-temperature and high-pressure phase-equilibria experiments. These previously published experimental results have been reinterpreted by taking explicit account of heat-capacity and entropy data for chamosite predicted from additivity models. In this way, major discrepancies in previously reported thermodynamic data for chamosite are resolved. -from Authors