New insights into the high-pressure polymorphism of SiO2 cristobalite

• Published in: Physics and chemistry of minerals Vol. 38; no. 7; pp. 517 - 529
• Main Authors: Dera, Przemyslaw, Lazarz, John D., Prakapenka, Vitali B., Barkley, Madison, Downs, Robert T.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer-Verlag 01.07.2011; Springer Nature B.V
• Subjects: Cristobalite; Crystal structure; Crystallography and Scattering Methods; Crystals; Densification; Earth and Environmental Science; Earth Sciences; Geochemistry; Metastable state; Mineral Resources; Mineralogy; Original Paper; Phase transitions; Polymorphism; Quantum mechanics; Silica; Silicon dioxide; Single crystals; Unit cell; X-ray diffraction; Phase transitions; Silica; High pressure; Metastability; Polymorphism
• ISSN: 0342-1791; 1432-2021
• DOI: 10.1007/s00269-011-0424-5

Single-crystal X-ray diffraction experiments with SiO 2 α-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 ~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 ~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.