Journal
PHYSICS AND CHEMISTRY OF MINERALS
Volume 38, Issue 7, Pages 517-529Publisher
SPRINGER
DOI: 10.1007/s00269-011-0424-5
Keywords
High pressure; Phase transitions; Metastability; Polymorphism; Silica
Categories
Funding
- National Science Foundation-Earth Sciences [EAR-0622171]
- Department of Energy-Geosciences [DE-FG02-94ER14466]
- U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-AC02-06CH11357]
- CDAC
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Single-crystal X-ray diffraction experiments with SiO2 alpha-cristobalite reveal that the well-known reversible displacive phase transition to cristobalite-II, which occurs at approximately 1.8 GPa, can be suppressed by rapid pressure increase, leading to an overpressurized metastable state, persisting to pressure as high as 10 GPa. In another, slow pressure increase experiment, the monoclinic high-pressure phase-II was observed to form at similar to 1.8 GPa, in agreement with earlier in situ studies, and its crystal structure has been unambiguously determined. Single-crystal data have been used to refine the structure models of both phases over the range of pressure up to the threshold of formation of cristobalite X-I at similar to 12 GPa, providing important constraints on the feasibility of the two competing silica densification models proposed in the literature, based on quantum mechanical calculations. Preliminary diffraction data obtained for cristobalite X-I reveal a monoclinic unit cell that contradicts the currently assumed model.
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