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Abstract. Twenty-one energy-dispersive X-ray diffraction spectra for fayalite at 400~ constitute the basis for an elevated-temperature static compression isotherm for this important silicate mineral. A Murnaghan regression of the resulting molar volumes yields 103.8 GPa and 7.1 for the 400 ~ C, room-pressure values of the isothermal bulk modulus (Ko) and its first pressure derivative (K;), respectively. When compared to the room-temperature static compression isotherm of Yagi et al. (1975), our 400 ~ C value for Ko yields -5.4

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We have derived an explicit, empirical V(T,P) equation based on an exponential temperature correction of the isothermal Murnaghan equation. The equation is symmetrical with respect to the first-and second-order derivatives of volume and is consistent with the thermodynamic relationship between (•)KT/•)T)p and (•)ctp/•)P)T. The only input data required are the five room temperature, room pressure parameters: Koo, K'oo, Ctoo, Ct'oo, and (3KT/3T)oo. We have successfully tested our equation against literature pressurevolume-temperature (P-V-T) data for two materials which exhibit quite different isochore behavior: NaC1 and S iO2 (o• quartz). For NaC1 our equation predicts isochores which are very nearly straight and parallel, reflecting the fact that the product ctpK T is approximately constant for this substance. For SiO2 our equation reproduces the strong negative curvature [(•2T/•PZ)v << 0] of the experimentally determinedisochores. These results suggest that our five-parameter equation can be used to accurately predict the P-V-T behavior of a wide variety of solids, including those for which isochore curvature is pronounced.

Abstract. Thirteen energy-dispersive x-ray diffraction spectra for y-Fe2SiO 4 (spinel) collected in situ at 400 ~ C and pressures to 24 GPa constitute the basis for an elevated-temperature static compression isotherm for this important high-pressure phase. A Murnaghan regression of these molar volume measurements yields 177.3 (+ 17.4) GPa and 5.4(+ 2.5) for the 400 ~ C, room pressure values of the isothermal bulk modulus (Kpo) and its first pres-K' sure derivative (po), respectively. When compared to the room-Tdeterminations of Keo available in the literature, our 400 ~ C Kpo yields -4.1 (_+6.2) x 10 -2 GPa/degree for the average value of (OK/OY)po over the temperature interval 25 ~ C < T < 400 ~ C.A five-parameter V(P, 7) equation for 7-Fe2SiO4 based on simultaneous regression of our data combined with the elevated P-Tdata of Yagi et al. (1987) (P, 7) relationships to the temperature and pressure of the 400 km discontinuity suggests a A Va of approximately 8.4% at that depth, approximately 10% less than the 9.3% AVR at ambient conditions.