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In situ falling-sphere measurements of viscosity have been performed to determine the viscosity of dacite melt (68 wt% Si[O.sub.2]) from 1.5 to 7.1 GPa at temperatures between 1730 and 1950 K. using the T-25 MA8 multianvil apparatus at the GSECARS 13 ID-D beamline at the Advanced Photon Source, Argonne National Lab. The viscosity of dacite melt decreases between 1.5 and 7.1 GPa. At 1.5 GPa and 1825 K the viscosity is 86.6 [+ or -] 17.3 Pa*s. whereas at 6.6 GPa and 1900 K it is 2.8 [+ or -] (1.6 Pa*s. The negative pressure dependence of viscosity results in an activation volume of -12.4 [+ or -] 1.4 [cm.sup.3]mol at 1800 K and -5.1 [+ or -] 0.9 [cm.sup.3]/mol at 1900 K. These new data are compared with viscosities estimated from the Eyring equation using oxygen sell diffusion data for the same bulk composition at high pressures. The Eyring equation generally predicts viscosities that are greater than measured viscosities. In addition, the Eyring equation predicts a minimum viscosity at 5 GPa, but no minimum was seen in our falling sphere data set. These discrepancies suggest that the mechanisms for viscous flow and self-diffusion of oxygen in polymerized melts may differ at high pressures, thus limiting the utility of the Eyring equation for high-pressure extrapolations. Further development of the Adam-Gibbs theory may provide an alternative for relating self-diffusion and viscosity at high pressures.

The yield strength of moissanite was investigated at pressures up to 18.3 GPa and temperatures up to 1200[degrees]C by analyzing the peak shapes of diffraction lines from a powder sample. At room temperature, the moissanite crystal behaves elastically with increasing pressure up to 13.7 GPa. At higher pressures applied, the sample is yielded; the yield strength of moissanite is determined to be 13.6 GPa. Upon heating at 18.3 GPa, significant stress relaxation is observed at temperatures above 400[degrees]C, and the yield strength of moissanite decreases rapidly from 12.8 GPa at 400[degrees]C to 4.2 GPa at 1200[degrees]C. Such behavior will place severe limitations on the use of moissanite as an anvil material when external heating is desired for high pressure and temperature experiments.

The deviatory stress of a compressed polycrystalline sample in Drickman-type apparatus is measured by X-ray diffraction. Diffracted energy-dispersive signals are measured simultaneously in two directions, relative to compression axis of the sample. The stress and strain components of the signals are determined using the alpha parameter and sigma(1)-sigma(3) uniaxial stress component.

The elastic properties and absolute pressure at high pressures and high temperatures in multianvil high-pressure experiment is determined. The pressure determined is believed to be more accurate measurement of the effective pressure imposed on the sample than the inferred from NaCl based on previous pressure scales.

The viscosity of liquid FeS and Fe-FeS eutectic was measured at pressures between 0.5 and 5.0 GPa using a synchrotron-based falling sphere technique. We obtain viscosities of 2 x [10.sup.-2] to 4 x [10.sup.-3] Pa-s in FeS at 1450 to 1700 [degrees] C and 2 x [10.sup.-2] to 8 x [10.sup.-3] Pa-s in Fe-[S.sub.eut], at 1150 to 1380 [degrees] C. These results are consistent with recent viscosity measurements in Fe-[S.sub.eut] at 5 to 7 GPa (Urakawa, in preparation), measured diffusivities (Dobson 2000) and ab initio simulated viscosity (Vocadlo et al. 2000). The results are also similar to the values for pure iron at low pressure (Shimoji and Itami 1986). A systematic increase in viscosity and activation energy is seen with increasing sulfur content. Interpolation between the data presented yields a viscosity of 1.4 x [10.sup.-2] Pa-s for an outer core composition with ~10 wt% S at the melting temperature. There is good evidence of homologous behavior for Fe-S liquids which implies that the liquid alloy at the inner core boundary may have a similar viscosity.