Pyrite-Type Silica Produced at High Pressures

Researchers at Tokyo Institute of Technology, the Japan Agency for Marine-Earth Science and Technology in Yokosuka and Japan Synchrotron Radiation Research Institute in Sayo have reported the observation of a form of silica with a pyrite-type crystal structure. They describe their findings, which have implications in planetary science, in the Aug. 5 issue of science.

Using a diamond anvil cell and an Nd:YLF laser, the investigators subjected samples of amorphous quartz or fused silica to various pressures and temperatures. During the experimental runs, they illuminated the samples with 0.41045- or 0.41296-Å monochromatic radiation to obtain angle-dispersive x-ray diffraction spectra.

Samples heated to a temperature of 1800 K at a pressure of 194 GPa and to temperatures of 1400 to 2000 K at pressures of between 220 and 257 GPa displayed no diffraction peaks indicative of the pyrite-type form. However, measurements of the sample subjected to temperatures of 1400 to 1800 K and to pressures of 268 to 271 GPa revealed a transition to the new crystal structure.

Although silica is plentiful in the Earth's lithosphere and mantle, the scientists note that pyrite-type silica requires pressures beyond those in these regions.

They suggest that conditions in the cores of ice giants such as Uranus and Neptune and in extrasolar planets may enable the phase transition to the pyrite-type form.