Abstract
The low-temperature heat capacities of natural near end-member minerals (about 95 mol%, except tinzenite of about 34 mol% on average) of the axinite group, previously characterized in detail by means of powder and single-crystal X-ray diffraction, electron microprobe, and Mössbauer spectroscopy, were measured by heat-pulse calorimetry using the Physical Properties Measurement System (Quantum Design) at temperatures between 5(2) and 300 K. From these data, the following entropy values [in J/(mol·K)] of the natural samples at 298.15 K were derived: S298,magnesioaxinite = 696.3 ± 1.1, S298,ferroaxinite = 743.5 ± 3.5, S298,manganaxinite = 737.5 ± 2.6, and S298,tinzenite = 758.1 ± 2.8. For the end-member compositions, the corrected heat capacities at 298.15 K and standard third-law entropies of the axinites are [all in J/(mol·K)]:
The standard entropies of manganaxinite and tinzenite include contributions of 1.9 and 4.3 J/(mol·K) for the range 0–5 K evaluated based on a Schottky anomaly fitted to the low-T CP values of these axinites. The lowest measured heat capacities of ferroaxinite indicate that a lambda-type CP anomaly should exist between 0 and 2 K. Its likely contribution to the standard entropy was estimated as ~5.2 J/(mol·K). A low-temperature CP anomaly below 15 K for ferroaxinite is well-explained by ferromagnetic ordering, whereas for manganaxinite by uncompensated antiferromagnetic ordering, and for tinzenite by pure antiferromagnetic ordering.
