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Large single crystals of stishovite were successfully synthesized at 11 GPa from a silica solution in water. The potential of both slow cooling and thermal gradient methods were examined. The thermal gradient method provided crystals of 0.8 x 0.8 x 1.3 mm in size grown at 1350 [degrees]C and a thermal gradient of 50 [degrees]C/mm using stishovite as a silica source. The use of quartz as a source resulted in the appearance of numerous stishovite crystals in the solution interior resulting in diminished space for the growth of large crystals. This can be explained by a significant difference in the solubility of metastable quartz and stishovite in water, estimated to be 85.3 and 5.6 wt% Si[O.sub.2] at 1000 [degrees]C and 11 GPa, respectively. Crystals up to 0.8 x 1.3 x 1.5 mm were grown by the slow cooling method in the system Si[O.sub.2] + 14.7 wt% [H.sub.2]O as temperature was decreased from 1600 to 1000 [degrees]C with a cooling rate of 2 [degrees]C/min. The size of single crystals obtained was large enough to carry out silicon self-diffusion experiments, which were performed at a pressure of 14 GPa and temperatures from 1400 to 1800 [degrees]C. The lattice diffusion coefficients along the [110] and [00l] directions can be expressed as [D.sub.[110]] ([m.sup.2]/s) 4.10 x [10.sup.-12] exp [-322 (kJ/mol)/RT] and [D.sub.[001]] ([m.sup.2]/s) = 5.62 x [10.sup.-12] exp [- 334 (kJ/mol)/RT], respectively, where R is the gas constant and T is the absolute temperature. Keywords: Stishovite, single-crystal growth, high pressure, aqueous fluid, silica solubility, diffusion DOI: 10.2138/am.2010.3255

Pressless split-sphere apparatus (BARS) equipped with an 8-6 type multi-anvil system is used extensively for large (6 carats) gem quality diamond crystal growth and various mineralogical and geochemical studies at shallow mantle conditions (P ≤ 7.5 GPa). In this study, we extend the pressure range for this apparatus to the conditions of the Earth's transition zone (P = 20 GPa) using a 6-8 multi-anvil system. A scaled-up Kawai-cell consisting of 47 mm WC cubic anvils with 10 mm truncation and 18 mm MgO pressure medium was employed for the experiments. A 300 mm spherical multi-anvil block compressed by a hydraulic system with 2500 atm capacity was used to compress the Kawai-cell. A pressure of 15.5 GPa was successfully generated at the oil pressure of 1875 atm. The comparison of the BARS apparatus calibrations with those obtained using a uniaxial split-sphere press USSA-5000 suggests that pressure up to 17 and 20 GPa in compressed cell/sample volumes of 1200/60 and 400/18 mm³, respectively, can be generated using a combination of the BARS apparatus and the scaled-up Kawai-cell. This cell volume is much larger than that in conventional multi-anvil systems, which does not exceed 80 mm3 at the same conditions. [ABSTRACT FROM AUTHOR]

Highlights: [•] Mg2SiO4 diffusivity in K–Mg carbonatite melt is 2×10−9 m2/s at 16.5GPa and 1700°C. [•] MgSiO3 diffusivity in hydrous K–Mg carbonatite melt is 4×10−9 m2/s at 24GPa, 1500°C. [•] MgSiO3 diffusivity in hydrous-silicate melt is 5×10−8 m2/s at 24GPa and 1500°C. [•] Stress-driven transcrystalline migration rate of TZ and LM melts is 10−1–102 m/year. [ABSTRACT FROM AUTHOR]