Pressure-Volume-Temperature equation of state of andradite and grossularia, by high-pressure and -temperature powder diffraction.

The minerals of the garnet group are major constituents of the upper mantle of the Earth, and play an important role in high pressure-high temperature petrogenetic processes. These minerals have a cubic cell, and an atomic structure formed by independent tetrahedra, octahedra and dodecahedra. The natural end member grossular (Ca3Al2Si3O12) and andradite (Ca3Fe2Si3O12) have been investigated at high temperature and high pressure conditions to determine their P-V-T EOS. Measurements have been performed on the ID30 beamline, by using a Large Volume Cell, over the following thermobaric range: from 300 to 1000 K, and from 0 to 6 GPa, by 0.3 GPa steps along isotherms at 300, 600, 800, 1000 K. Temperature has been monitored by a Pt-thermocouple, and pressure estimated by NaCl as an internal calibrant. A total of 120 diffraction profiles have been collected for both samples, and processed by multi-phase profile analysis according to the LeBail et al. (1988) method, as implemented in the GSAS sofware package (Larson and von Dreel, 1987). Fig. 1a and Fig.1b report the cell parameters versus P for andradite and grossular, respectively. The experimental P-V-T data have been intepolated by means of the Birch-Murnaghan equation of state modified for the high temperature regime. The bulk modulus Ko and the thermal expansion (a) have been determinated. Grossular: Ko= 168.1 (±1.4) GPa, K’o=4.0, a=25.7(3) 10-6K-1; andradite: Ko=157.8 (±1.7) GPa, K’o=4.0, a=31.4 (3) 10-6K-1.