Moissanite

Abstract: We achieved a pressure of 52.1 gigapascals with moissanite anvils, which have optical, thermal, electric, magnetic, and x-ray properties that rival those of diamond. The mode-softening of D 2 O toward the pressure-induced hydrogen bond symmetrization and the Raman shifts of diamond under hydrostatic and nonhydrostatic compressions were studied with moissanite anvils in the spectral regions normally obscured by diamond anvils. Moissanite anvil cells allow maximum sample volumes 1000 times larger than those allowed by diamond anvil cells and may enable the next level of advancement in high-pressure experiments.

Abstract: The evolution of texture and crystal size distribution of igneous rocks is mainly determined by processes such as nucleation, grain growth, and Ostwald ripening. However, the role played by these processes on the final crystal size distribution is difficult to quantify with conventional methods. Here we present the first direct in situ observation of a crystallizing supercooled basaltic-andesitic liquid in order to directly assess the role of grain-scale processes occurring during magma solidification. The experiments were conducted at 900 °C and 1 bar with a new moissanite cell specifically designed for long-term high-temperature experiments. The method allowed the direct continuous observation of the crystallizing melt until the crystal fraction approached 0.7. Investigation of the evolving texture revealed that individual crystals followed distinct growth histories characterized by intervals of relatively uniform free growth, abrupt size increase due to grain coalescence, and growth reduction due to impingement; the latter played a dominant role at higher crystal fractions. Despite this growth dispersion, average growth rates calculated for single grains showed only little variation (4.9 × 10−6 to 1.9 × 10−6 mm/s). No clear relation between growth rate and crystal size was observed. Crystal coarsening through a combination of overgrowth and coalescence can also account for the counterclockwise rotation and the drop-off at smaller size fractions of the recovered crystal size distributions in our experiments. These results suggest that crystal size distributions of natural rocks do not record the original nucleation rates.