New high-pressure transformation in α-quartz

Abstract

Recent experimental and theoretical studies report that quenched pressure-amorphized a-quartz is elastically anisotropic and displays a reversal of anisotropy with respect to the original crystallographic orientation. We demonstrate that samples recovered from static compression experiments of a-quartz exhibit an unusual pressure-induced microstructure, which arises from a new phase transformation in aquartz at 21 GPa. Upon decompression, the high-pressure phase reverts to a quartzlike structure in a twinned state, accounting for the previously reported elastic behavior.

Theoretical study of a five-coordinated silica polymorph

“…3 ␣-quartz, the stable form at ambient conditions, undergoes a crystalline-to-crystalline transition at 21 GPa prior to pressure-induced amorphization. [4][5][6][7] Similar transformations have been observed for cristobalite and for the isomorphic forms of quartz, GeO 2 and AlPO 4 . [8][9][10][11] These transitions are associated with the formation of tetrahedral networks, and at higher pressure, increases in cation coordination number.…”

“…78 This transition pressure is close to that of the experimentally reported crystalline-crystalline phase transition documented in high-pressure hydrostatic experiments of quartz. 7 The present study strongly suggests that when quartz is compressed in a nonhydrostatic manner with the stress along c greater than that along a and b, this instability is postponed to higher pressures. The resulting strain allows the silicon atoms to experience a different oxygen environment than would normally be encountered under hydrostatic conditions.…”

“…Studies of metastable transitions in silica, and quartz in particular, produce heterogeneous samples, with small amounts of crystalline ͑or partially crystalline material͒ mixed with material having disorder ranging from extensive twinning to completely amorphous. 6,7 Both vibrational and diffraction measurements yield broadbands. Deviatoric stresses are present to some degree in most experimental studies: Under such conditions, local stresses within the heterogeneous sample are expected to be significant, even if the sample assemblage is in a hydrostatic environment, because of local grain-grain-contacts.…”

Second harmonic generation measurements at high pressures on powder samples

“…However, a single crystal X-ray diffraction study of quartz (in argon) showed no pressure-induced amorphization up to 19.3 GPa [31]. At 21 GPa, the quartz II phase and new lines are observed in a nonhydrostatic pressure medium [32,33]. The quartz II and the new phase with space group P2 1 =c appeared around 19-21 GPa in a quasihydrostatic pressure medium and both phases persisted stably up to 40 GPa [35].…”

Investigation of pressure-induced amorphization in hydrated zeolite Li-A and Na-A using synchrotron X-ray diffraction

“…The observed pressureinduced amorphization of scolecite and mesolite has been attributed to the increased spreading of the T±O±T angles caused by the progressive disruption of 4-member rings. This process has been observed in other materials [10,20,21] and could be the cause of pressure-induced amorphization of zeolite Li-A and Na-A. However, since the unit cell volume for scolecite is much smaller than that of Linde A and the pore and channels in the Linde A zeolites are much larger and more extensive, a comparison between the amorphization pressures is limited.…”