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

Jan Filip^1,2,*, Edgar Dachs³, Jií Tuek^1,4, Milan Novák² and Petr Bezdika⁵

¹ Centre for Nanomaterial Research, Palack University in Olomouc, Svobody 26, 771 46 Olomouc, Czech Republic
² Department of Geological Sciences, Masaryk University, Kotláská 2, 611 37 Brno, Czech Republic
³ Department of Material Engineering and Physics, University of Salzburg, Hellbrunnerstrasse 34, 5020 Salzburg, Austria
⁴ Department of Experimental Physics, Palack University in Olomouc, Svobody 26, 771 46 Olomouc, Czech Republic
⁵ Institute of Inorganic Chemistry, Academy of Science of the Czech Republic, 250 68 e, Czech Republic

Correspondence: ^* E-mail: jan.filip{at}upol.cz

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)]:

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.

Key Words: Axinite group minerals • chemical composition • heat capacity • heat-pulse calorimetry • magnetic ordering • low-temperature anomaly