Thermodynamic properties of magnesiochloritoid

Calorimetric and P-V-T data of synthetic magnesiochloritoid (MgAl 2 SiO 5 (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 A 3 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 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·B 2 O 3 ) 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 MgAl 2 SiO 5 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-Muller 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 −1 mol −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 Δ f H 0 298 (magnesiochloritoid) = −3551.7 kJ mol −1 , and S 0 298 (magnesiochloritoid) = 142.2 J K −1 mol −1 .