We report new data on the melting curve of
H2O
in the range 20–90 GPa and 1000–2400 K obtained in a laser-heated diamond anvil cell. We
found a marked discrepancy between our present results and previous work which
covered the pressure range up to 38 GPa. The melting curve shows a discontinuous
change in slope at about 43 GPa, indicating a first-order phase transition most
likely to the ice X phase. We could find no evidence for a strong dissociation of
H2O
up to 200 K above the melting temperatures.
It had long been accepted that the 400-km seismic discontinuity in the Earth's mantle results from the phase transition of (Mg,Fe)2-SiO4-olivine to its high-pressure polymorph beta-spinel (wadsleyite), and that the 660-km discontinuity results from the breakdown of the higher-pressure polymorph gamma-spinel (ringwoodite) to MgSiO3-perovskite and (Mg,Fe)O-magnesiowüstite. An in situ multi-anvil-press X-ray study indicated, however, that the phase boundary of the latter transition occurs at pressures 2 GPa lower than had been found in earlier studies using multi-anvil recovery experiments and laser-heated diamond-anvil cells. Such a lower-pressure phase boundary would be irreconcilable with the accuracy of seismic measurements of the 660-km discontinuity, and would thus require a mineral composition of the mantle that is significantly different from what is currently thought. Here, however, we present measurements made with a laser-heated diamond-anvil cell which indicate that gamma-Mg2SiO4 is stable up to pressure and temperature conditions equivalent to 660-km depth in the Earth's mantle (24 GPa and 1,900 K) and then breaks down into MgSiO3-perovskite and MgO (periclase). We paid special attention to pressure accuracy and thermal pressure in our experiments, and to ensuring that our experiments were performed under nearly hydrostatic, inert pressure conditions using a variety of heating methods. We infer that these factors are responsible for the different results obtained in our experiments compared to the in situ multi-anvil-press study.
High-pressure X-ray diffraction studies on BaFCl and BaFBr up to 50 GPa, on SrFCl up to 42 GPa, and on CaFCl up to 27 GPa were performed at room temperature using a diamond anvil cell and synchrotron radiation. Data for the equations of state are given. The c/a ratios of the tetragonal structure of these compounds show different variations under pressure and structural phase transitions are observed for BaFCl and BaFBr at transition pressures of about 21 GPa and 27 GPa, respectively. A simple hard-sphere model is presented, which describes the observed structural variations almost quantitatively and accounts for the structural phase transition.
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