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Thermodynamic properties of magnesiochloritoid

¹ Ruhr-Universität Bochum, Institut für Geologie, Mineralogie und Geophysik, D-44780 Bochum, Germany
² Thermochemistry Facility and NEAT ORU, University of California at Davis, Davis, CA 95616–8779, U.S.A.
³ now at: Institut für Mineralogie und Geochemie, Albert-Ludwig-Universität Freiburg, Albertstrasse 23b, D-79104 Freiburg, Germany
⁴ GeoForschungsZentrum Potsdam, Department 4, Telegrafenberg, D-14473 Potsdam

^* E-mail: Klaus-Dieter.Grevel{at}ruhr-uni-bochum.de

Calorimetric and P-V-T data of synthetic magnesiochloritoid (MgAl₂SiO₅(OH)₂) have been obtained. The P-V-T behaviour of monoclinic and triclinic magnesiochloritoid has been determined in situ up to 8.5 GPa and 800°C using a MAX 80 cubic anvil high-pressure apparatus. The samples were mixed with vaseline to ensure hydrostatic pressure transmitting conditions; NaCl served as an internal standard for pressure calibration. By fitting a Birch-Murnaghan EOS to the data, the bulk modulus of the triclinic polytype was determined as 127.9 ± 2.1 GPa, (K’ = 4), V_T,0 = 456.58 ųexp [(0.304 ± 0.022) x 10^–4 dT], (K_T/T)_P = –0.017 ± 0.009 GPa K^–1. The resulting fit parameters for the monoclinic polytype are very similar.

The enthalpy of drop-solution was measured by high-temperature oxide melt calorimetry in two laboratories (UC Davis, California, and Ruhr-University Bochum, Germany) using lead borate (2 PbO·B₂O₃) at 700°C as solvent. The resulting values were used to calculate the enthalpy of formation from the elements;-3538.9 ± 4.9 kJ mol^–1 (Davis) and –3543.4 ± 6.2 kJ mol^–1 (Bochum) were obtained.

Heat capacity measurements of MgAl₂SiO₅ were obtained by differential scanning calorimetry (DSC) in the temperature range from –10°C to 295°C. Two runs confirmed heat capacity data of Koch-Müller et al. (2002), represented by the Berman & Brown (1985) type four-term equation: C_P = (391.75 - 2585.00 x T^–0.5 - 8240000.0 x T^–2 + 967000000.0 x T^–3) J K^–1mol^–1.

Consistency of the thermodynamic data obtained for magnesiochloritoid with phase equilibrium data reported in the literature was checked by mathematical programming analysis. The best agreement was obtained with _fH⁰₂₉₈ (magnesiochloritoid) = –3551.7 kJ mol^–1, and S⁰₂₉₈ (magnesiochloritoid) = 142.2 J K^–1mol^–1.

Key-words: thermodynamics, calorimetry, high-pressure, magnesiochloritoid.

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