THERMODYNAMIC PROPERTIES AND HYDROGEN SPECIATION FROM VIBRATIONAL-SPECTRA OF DENSE HYDROUS MAGNESIUM SILICATES

Infrared absorption measurements were taken from 100 to 5000 cm(-1) of a natural chondrodite and three dense hydrous magnesium silicates: ph ase A, phase B, and superhydrous phase B (shy-B). Raman spectra were a lso acquired from phase B and the chondrodite. Roughly half of the lat tice modes are represented and our data are the first report of the lo w frequency modes. Comparison of our new spectra to symmetry analyses suggests that multiple sites for hydrogen exist for all the phases. Th e shy-B we examined crystallizes in P2(1)nm with two OH sites. Models for the density of states are constructed based on band assignments fo r the lattice modes and for the OH stretching vibrations. Heat capacit y C-p and entropy S calculated using Kieffer's formulation should be a ccurate within 3% from 200 to 800 K. Model values for C-p at 298 K are 299.6 J/mol-K for chondrodite, 421.5 J/mol-K for phase A, 529.4 J/mol -K for shy-B, and 618.9 J/mol-K for phase B. Model values for S-298(0) are 234.2 J/mol-K for chondrodite, 303.5 J/mol-K for phase A, 377.9 J /mol-K for shy-B, and 473.3 J/mol-K for phase B. Debye temperatures ar e near 1000 K.