Pressure-induced transformations of cristobalite

Discovery of seifertite in a shocked lunar meteorite

“…TEM observations show that cristobalite had transformed to seifertite near the shock veins, whereas cristobalite had transformed to stishovite farther from them; that is, cristobalite transformed to seifertite in hotter portions, but to stishovite in colder portions under the same pressure. These occurrences imply that the Clapeyron slope between stishovite and seifertite is negative, even though there is some disagreement, based on deductions from synthetic experiments, about the Clapeyron slope between stishovite and seifertite (or CaCl 2 -type silica) 12,25,26 . Cristobalite becomes unstable as pressure and temperature increase in a dynamic event.…”

High pressure induced phase transformation of SiO2 and GeO2: difference and similarity

“…There are two well established polymorphs of GeO 2 at ambient conditions: a-quartz-type structure and rutile-type structure with fourfold-and sixfold-coordinated germanium ions, respectively. However, there are inconsistencies in the results of high pressure phase transformations of GeO 2 reported by different groups. The amorphous structure of the high pressure phase of a-quartz type GeO 2 above 6 GPa has been reported by several groups [10 -14].…”

“…The new data together with previous results for SiO 2 and GeO 2 are compared, and polymorph sequences are discussed.…”

“…At ambient pressure, SiO 2 has four major polymorphs: cristobalite (low temperature tetragonal, S.G.: P4 1 2 1 2 and high temperature cubic, S.G.: Fd-3m), tridymite (hexagonal, S.G.: P6 3 /mmc), quartz (low temperature (a-quartz) trigonal, S.G.: P3 2 21 and high temperature (b-quartz) hexagonal, S.G.: P6 2 22) and glass (amorphous network). There are a number of theoretical and experimental reports concerning the behavior of various high pressure silica phases: keatite, coesite, stishovite, CaCl 2 -, a-PbO 2 -, I2/a, baddeleyite, fluorite, and pyrite(Pa-3)-types.…”

“…However, the experimental evidence of phase boundaries is still not clear, and some reported results were contradictory among different groups on phase transformations at high pressures. For example, the phase boundary between stishovite and a-PbO 2 type phase was observed at , 80 GPa and 2500 K [2] when cristobalite was used as a starting material. Meanwhile, no evidence was found for a further transformation of the CaCl 2 -type structure up to 128 GPa using stishovite as a starting material [3,4].…”