Thermodynamic properties of magnesiochloritoid

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 ų exp [∫(0.304 ± 0.022) × 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 enthal py of drop-solution was measured by high-temperature oxide melt calorimetry in two laboratories (UC Davis, California, nd 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(OH)₂ 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 × T^-0.5 - 8240000.0 × T^-2 + 967000000.0 × T^-3) J K^-1mol^-1. Consistenc y 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 Δ_fH⁰₂₉₈ (magnesiochloritoid) = -3551.7 kJ mol^-1, and S⁰₂₉₈ (magnesiochloritoid) = 142.2 J K^-1mol^-1.