A discussion on the stability of ferroactinolite : Experimentation vs. estimation

Ferroactinolite is an important phase belonging to the Ca-Fe-Si-O-H system. Since late sixties, only one experimental work constraining its stability within the P-T-f02 volume is available. From this work, many authors have tried to retrieve the thermodynamic parameters of this mineral but the agreement among the different estimations is, in general, poor. We have compiled what we believe the most reliable volumetric data for ferroactinolite and we have calculated Its high temperature heat capacity as well as its 3rd Law entropy from additivity constraints. Our data, coupled with experimental P-T brackets and the thermodynamic parameters of magnetite, quartz and fayalite, could allow us to estimate the standard Gibbs free energy of this mineral at the reference temperature and pressure. Unfortunately, there is no agreement between the constrained experimental brackets and our theoretical calculations so the estimated Gibbs free energy value is not satisfactorily constrained. The available experiments appear to be highly inaccurate due probably to the bad crystallochemical characterization of reactants and run products and, therefore, it would be highly desirable to perform new and hotter constrained experiments in order to fully characterize this important field of the Ca-Fe-Si-O-H system and its geological implication