Pressure-Induced Transformations in Silica

Polymorphic phase transitions in the silica minerals alpha-quartz and stishovite have been investigated using diamond-anvil cell techniques at room temperature. Structural and vibrational properties of these materials were monitored as a function of pressure using in situ Raman scattering, synchrotron x-ray diffraction, and optical microscopy. Pressure-quenched samples were characterized at ambient conditions using Raman spectroscopy, electron diffraction, transmission electron microscopy, backscattered and secondary electron imaging, and optical microscopy. Solid-state amorphization of alpha -quartz has been found to begin with formation of crystallographically controlled planar defects, followed by growth of amorphous silica at these defect sites. Characteristic microstructures (planar defects and amorphous lamellae) are found in quartz upon quasihydrostatic and nonhydrostatic compression and from comminution, suggesting that there is a common mechanism for solid-state amorphization of silicates in static and shock compression experiments, meteorite impact, and deformation by tectonic processes. A new crystalline-crystalline transformation has been discovered in alpha-quartz at 21 GPa, documented by abrupt changes in the synchrotron x-ray diffraction pattern and the Raman spectrum. Upon decompression, the high-pressure phase reverts to a quartz -like structure in an unusual twinned state. The Raman spectrum of samples recovered from hydrostatic compression closely resembles spectra of both dynamically shocked quartz and quartz that has experienced extensive grinding; each shows significant deviations from the spectrum of pristine quartz. The transformation from rutile-structured silica (stishovite) to the CaCl_2-structured form has been documented by high-pressure Raman scattering at 51 GPa. At this pressure, the pressure dependence of the soft B_{1rm g} vibrational mode changes sign, and the stishovite E _{rm g} mode splits, as predicted for the transformation to the CaCl_2 structure. The spectral changes are continuous and reversible with no hysteresis, suggesting that the transformation is second-order. Finally, the Raman spectrum of the recently documented new silica phase "moganite" has been measured at ambient conditions. Comparison with Raman spectra of other tetrahedrally coordinated minerals confirms that "moganite" is a structurally distinct polymorph of silica.