Low-temperature heat capacities and derived thermodynamic properties of anthophyllite, diopside, enstatite, bronzite, and wollastonite

The heat capacities for magnesio-anthophyllite, diopside, synthetic enstatite, bronzite, and wollastonite were measured between 5 and 385 K by use of an adiabatic calorimeter. The entropy change $S298o−S0o$⁠, in J/(mol · K), is 538.9±2.7 for magnesio-anthophyllite [Mg_6.3Fe_0.7Si₈O₂₂(OH)₂], 142.7±0.2 for diopside, 66.27±0.10 for synthetic enstatite (MgSiO₃), 69.04±0.10 for bronzite (Mg_0.85Fe_0.15SiO₃), and 81.69±0.12 for wollastonite. The heat capacity, $CPo$⁠, for magnesio-anthophyllite, corrected to a composition of pure Mg-anthophyllite [Mg₇Si₈O₂₂(OH)₂], results in a $S298o−S0o$ value of 537.0±2.7 J/(mol·K). These results represent only the entropy calculated from the measured heat capacities. No configurational entropy was added.

Although our $CPo$ values for diopside and wollastonite differ significantly from those of previous studies, especially at cryogenic temperatures, our entropies at 298.15 K are in close agreement with the commonly accepted values. Schottky heat capacity anomalies were observed below 25 K for magnesio-anthophyllite, diopside, and bronzite. The contribution of the magnetic entropy arising from the interaction of Fe²⁺ with the ligand field of the crystals is discussed.