Thermodynamic properties of magnesiochloritoid

Calorimetric and P-V-T data of synthetic magnesiochloritoid (MgAl (sub 2) SiO (sub 5) (OH) (sub 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 degrees 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 + or - 2.1 GPa, (Ka' = 4), V (sub T,0) = 456.58 A (super 3) exp [Sigma (0.304 + or - 0.022) X 10 (super -4) dT], ((partial differential)K (sub T) /(partial differential)T) (sub P) = -0.017 + or - 0.009 GPa K (super -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 (sub 2) O (sub 3) ) at 700 degrees C as solvent. The resulting values were used to calculate the enthalpy of formation from the elements; -3538.9 + or - 4.9 kJ mol (super -1) (Davis) and -3543.4 + or - 6.2 kJ mol (super -1) (Bochum) were obtained. Heat capacity measurements of MgAl (sub 2) SiO (sub 5) (OH) (sub 2) were obtained by differential scanning calorimetry (DSC) in the temperature range from -10 degrees C to 295 degrees C. Two runs confirmed heat capacity data of Koch-Muller et al. (2002), represented by the Berman & Brown (1985) type four-term equation: C (sub P) = (391.75 -2585.00 X T (super -0.5) -8240000.0 X T (super -2) + 967000000.0 X T (super -3) ) J K (super -1) mol (super -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 Delta (sub f) H (super 0) (sub 298) (magnesiochloritoid) = -3551.7 kJ mol (super -1) , and S (super 0) (sub 298) (magnesiochloritoid) = 142.2 J K (super -1) mol (super -1) .