Low-temperature calorimetric and magnetic data for natural end-members of the axinite group

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: S 298,magnesioaxinite = 696.3 ± 1.1, S 298,ferroaxinite = 743.5 ± 3.5, S 298,manganaxinite = 737.5 ± 2.6, and S 298,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)]: CP o magnesioaxinite = 827.5 ± 1.5 and S o magnesioaxinite = 693.7 ± 1.1, CP o ferroaxinite = 841.8 ± 3.3 and S o ferroaxinite = 749.6 ± 3.5, CP o manganaxinite = 849.1 ± 2.5 and S o manganaxinite = 737.8 ± 2.6, and CP o tinzenite = 841.6 ± 2.6, S o tinzenite = 754.0 ± 2.8. 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 C P values of these axinites. The lowest measured heat capacities of ferroaxinite indicate that a lambda-type C P 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 C P 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.