Re-examination of the run products in the Mg₂SiO₄–H₂O system by Ohtani et al. (1995) revealed stability of dense hydrous magnesium silicate, phase G, and brucite at 20 GPa and 800°C, and stability of the assemblage with superhydrous phase B + phase G + fluid at 20 GPa and 1000°C under the water saturated conditions. We reported the Raman spectrum of phase G, which suggests existence of silicon ions in the six coordination sites. Brucite, superhydrous phase B, and phase G are stable at least up to 22.5 GPa and around 1000°C, and are the candidates for the hydrous phases in a cold slab descending into the base of the transition zone and the uppermost part of the lower mantle. The X-ray diffraction and the Raman spectra data revealed that the dense hydrous magnesium silicate, phase G, and phase D re-defined by Yang et al. (1997) and Kuroda and Irifune (1998) are identical. Phase F by Kanzaki (1991) and Gasparik (1993) is likely to be identical to phase G.

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Energy-minimization calculations with accurate transferable interatomic potentials have been performed to investigate the possible existence of two new high-pressure aluminous phases, MgAl₂O₄ with the CaFe₂O₄ structure (called HPCF) and Al₂SiO₅ with the V₃O₅ structure (HPVO), both proposed previously based on high-temperature and high-pressure experiments. Of these, the HPCF phase is found to be energetically stable with reasonable nearest-neighbour bond distances. The calculated static compression of HPCF also agrees very well with measured data. In addition the calculated X-ray powder intensities based on the energy-minimized structural parameters for the HPCF phase compare well with experiment. Thus the present energy calculation confirms the existence of the HPCF phase with the CaFe₂O₄ structure. On the other hand, it is found to be energetically very unlikely that the HPVO phase has the V₃O₅ structure. The X-ray powder pattern for this phase, calculated based on the energy minimized structural parameters assuming the V3O₅ structure, shows no resemblance to experimental data. Hence, the existence of the previously suggested HPVO phase with the V₃O₅ structure is very questionable.

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Giessenite was reported as an acicular crystal that shared a close association with Sb-bearing cosalite from the Otome mine, Yamanashi Prefecture (Ozawa and Hori, 1982). Recent criteria classify it as izoklakeite.
The Otome izoklakeite, which is orthorhombic (Pnnm or Pnn2 from its orthogonal lattice and systematic extinction, in agreement with previous reports), has unit-cell dimensions of a 34.067, b 38.085, c 4.056Å. In addition, diffuse reflections which double the periodicity of 4.056Å along the c-axis are observed, also in agreement with the report on Vena izoklakeite (Zakrzewski and Makovicky, 1986).
An electron microprobe analysis gives Cu 0.8, Fe 0.3, Ag 1.1, Pb 46.5, Bi 27.2, Sb 7.3, S 16.2, total 99.4wt.%, yielding the empirical formula (Cu_2.7Fe_1.2)_3.9Ag_2.2Pb_48.7(Bi_28.2Sb_13.0)_41.2S_109.7, assuming the total cation=96 in a unit cell. It has a Sb/(Sb+Bi) value of 0.316, and is the most Bi-rich of the known izoklakeites.

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Potential parameters for molecular dynamics (MD) simulations in the K₂O–Na₂O–CaO–MgO–Al₂O₃–SiO₂ system were newly evaluated and applied successfully to reproduce the cell parameters and the symmetries of 25 crystals at 300K and 1 atm and the thermal behaviour of plagioclase feldspar.

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