Growth of large MgSiO3 perovskite and SiO2 stishovite single crystals

Knowledge of the physical properties of mantle material is essential for understanding the structure and dynamics of our planet. Large perfect single crystal samples of mantle minerals are necessary for the accurate determination of their physical properties, such as elasticity, plasticity, electrical and thermal conductivity. In this study, we describe an experimental technique to perfect the crystal growth of perovskite and stishovite, and highlight our success in synthesizing single crystals of 1 mm in size. Experiments were performed using a Kawai-type high pressure apparatus (USSA-5000). The method of crystal growth from solution in the thermal gradient field (Wentorf 1971; Pal'yanov et al. 1997; Sumiya et al. 2005) was employed at 1500°C, 24 and 12 GPa for perovskite and stishovite, respectively. We found that huge thermal gradient of 150°C/mm, which is common for ultra-high pressure experiments, causes of high supersaturation and so high nucleation density of desirable phases. Since crystal growth is hindered by neighboring crystals, the nucleation density was suppressed by the small thermal gradient of 20°C/mm, permitting an increase in the free space for large crystal growth. Solvent with high silicate solubility and low viscosity is needed for growth of high quality crystals. For perovskite we applied H2O and KHCO3-Mg(OH)2 solvents and MgSiO3 silicate source, which able us to grow 1-mm size crystals. However, in the both cases crystals contain stishovite inclusions, due to incongruent dissolution of perovskite in the solvents. Moreover, in the carbonate solvent the crystals contain melt inclusions. The composition of the melt, which is equilibrium with perovskite in MgO-SiO2-H2O system at 24 GPa and 1500°C, was SiO2 - 37.2wt%, MgO - 42.1wt%, H2O - 20.7wt%. Later, to prevent inclusion trapping, we used this composition as solvent, which allowed us to get inclusion free single crystals more than 1 mm in size. KHCO3 and KHCO3-Mg(OH)2 solvents and SiO2 silicate source were applied for stishovite crystal growth. In the first system potassium-wadeite (K2Si4O9) crystallized instead of stishovite. In the second system 1-mm stishovite crystals were successfully synthesized. From known weight of starting materials and stishovite crystals we determined solubility of stishovite in the K2Mg(CO3)2+13.9wt% H2O melt, which achieved 20wt%. The use of equilibrium melt as a solvent permitted the crystal growth on oriented seed crystals.