We report in situ density values of amorphous ice obtained between 0.3 and 1.9 GPa and 144 to 183 K. Starting from high-density amorphous ice made by pressure-amorphizing hexagonal ice at 77 K, samples were heated at a constant pressure until crystallization to high-pressure ices occurred. Densities of amorphous ice were calculated from those of high-pressure ice mixtures and the volume change on crystallization. In the density versus pressure plot a pronounced change of slope occurs at approximately 0.8 GPa, with a slope of 0.21 g cm(-3) GPa(-1) below 0.8 GPa and a slope of 0.10 g cm(-3) GPa(-1) above 0.8 GPa. Both X-ray diffractograms and Raman spectra of recovered samples show that major structural changes occur up to approximately 0.8 GPa, developing towards those of very high-density amorphous ice reported by (T. Loerting, C. Salzmann, I. Kohl, E. Mayer and A. Hallbrucker, Phys. Chem. Chem. Phys., 2001, 3, 5355) and that further increase of pressure has only a minor effect. In addition, the effect of annealing temperature (T(A)) at a given pressure on the structural changes was studied by Raman spectra of recovered samples in the coupled O-H and decoupled O-D stretching band region: at 0.5 GPa structural changes are observed between approximately 100-116 K, at 1.17 GPa between approximately 121-130 K. Further increase of T(A) or of annealing time has no effect, thus indicating that the samples are fully relaxed. We conclude that mainly irreversible structural changes between 0.3 to approximately 0.8 GPa lead to the pronounced increase in density, whereas above approximately 0.8 GPa the density increase is dominated to a large extent by reversible elastic compression. These results seem consistent with simulation studies by (R. Martonàk, D. Donadio and M. Parrinello, J. Chem. Phys., 2005, 122, 134501) where substantial reconstruction of the topology of the hydrogen bonded network and changes in the ring statistics from e.g. mainly six-membered to mainly nine-membered rings were observed on pressure increase up to 0.9 GPa and further pressure increase had little effect.

The phase relations of low quartz, high quartz, and coesite were reinvestigated using extremely pure SiO2. The study was conducted with a piston-cylinder apparatus employing low-friction cells. While the low quartz-coesite transition between 600 ø and 1100øC was determined by conventional quench techniques, the greater portion of this study relied on in situ methods (differential thermal analysis and a volumetric method referred to as differential pressure analysis). The agreement with previous results is good; however, the precision of present data is considerably higher. A comparison of our salt-cell data with the raw data of previous studies shows a pressure loss due to friction of about 10% for talc cells. A discussion of thermochemical aspects of the low quartz-coesite transition indicates improved agreement between thermodynamic and experimental data. Reconnaissance data on the influence of the NaAIO2 component show above 600øC in the absence of H20 a considerable shift of the quartz-coesite boundary toward higher pressure (4 kbar at 1100øC). In the presence of H20 a partial liquid is formed, and the shift is reversed; the transition boundary is reduced 3 kbar and lies between 680 ø and 1000øC at even lower pressure than that of the pure SiO2 modifications.

The system CaSO(4)-H(2)O, characterized by the three dehydration reactions gypsum-anhydrite, gypsum-bassanite, and bassanite-anhydrite, was reexamined by in situ differential pressure analysis in the temperature range of 60-350 degrees C up to 3.5 GPa pressure. The investigation revealed a fine structure in the dehydration boundaries of gypsum-bassanite and bassanite-anhydrite, each characterized by three inflections at 0.9-1.0, 1.9-2.0, and 2.6-28 GPa. In addition, the phase transition of anhydrite high pressure anhydrite (monazite structure) was established for the first time at high P-T conditions intersecting the bassanite-anhydrite dehydration boundary at 2.15 GPa250 degrees C. Furthermore, the triple point gypsum-bassanite-anhydrite was redetermined with 235 MPa80.5 degrees C. The evaluation of the gypsum-bassanite dehydration boundary with respect to the volume and entropy change of the reaction, DeltaV(react) and DeltaS(react), by means of the Clausius-Clapeyron relation yields for the entropy parameter an unusually large increase over the range of the noted inflections. This is interpreted as anomalous entropy behavior of H(2)O related presumably to a dramatic increase in fluctuations of the hydrogen network of the liquid leading possibly into a new structural state. The effect is strongly related to the three noted pressure levels of 0.9-1.0, 1.9-2.0, and 2.6-28 GPa. In a synopsis of data including also a previous high pressure study in the temperature range between 0 and 80 degrees C, a tentative P-T diagram of H(2)O is proposed.

The melting curves of gold, silver, and copper were redetermined by differential thermal analysis to 60-kbar pressure. The experiments were carried out with an end-loaded piston-cylinder high-pressure apparatus using a recently developed low-friction cell. Significant improvements in the precisionof the temperature determination above 40 kbar were obtained.We observed a slight lowering of the melting temperature of copper when graphite capsules were used, which is consistent with our past experience and indicates some alloying of the copper and carbon.The following parabolic functions (T m = a 3 + [a2(P-a•)]l/2 10 8a 2) were fitted to the raw data, and •he data corrected for the influence of pressure on the emf of the thermocouples (in parenthesis): gold T m = -2439.28+[0.262850x10-4(p+322.668)]l•2 x 2628.50 (T m = -1773.88+[0.309303x10-4(p+249.167)]l/2 x 3093.03); silver, T m =-261.759+[0.661750x10 -4 (P+99.085)]1/2 x 6617.50 (T m = -194.145+[0.668583 x 10-4(p+89.347)]1/2 x 6685.83); and copper, Tm= 129.171+[0.134011x10-3(p+122.305)]l/2 x 134.011 (T m = 199.265+[0.135218x10-3(P+105.960)]l/2 x 135.218).The precision of the pressure measurements at the 90% confidence level is 0.3+-0.5 kbar. The precision in the temperature is e5øC.

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