Stishovite and Other Six-Fold Coordinated Structures in Silica and Related Compounds
Abstract
The discovery of stishovite opened up the possibility of silicon in octahedral coordination in silica. Since this discovery, several other dense silica forms with six-fold coordination have been found such as, for example, CaCl2-, alpha-PbO2- and Fe2N-type silica. These structures are typically based on various cation-ordered superstructures of disordered Fe2N-type silica. These phases can be classed as either stable high-pressure, post-stishovite forms or purely metastable structures. One such metastable form is the new monoclinic polymorph that we have very recently obtained by rapid compression of alpha-quartz to 45 GPa at room temperature in a close to hydrostatic, helium pressure medium. The x-ray diffraction data obtained could be indexed based on a monoclinic cell and the intensities are in agreement with a P21/c model structure built up of 3 X 2 zigzag chains of SiO6 octahedra. This new polymorph of silica, which is metastable under ambient conditions, has been isolated for the first time and is one of several possible competing dense forms containing octahedrally coordinated silicon. The relative stability of these different forms with respect to stishovite will be discussed and compared to the results in silica analogs, in particular, germanium dioxide. The experimental results vary strongly depending on the nature of the starting material used and the observed phase transition sequences will be compared with theory. This rich crystal chemistry of the dense structures of silica and the place of this new polymorph in relation to that of stishovite and other six-coordinated forms will be highlighted.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.T22E..08H
- Keywords:
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- 3620 Crystal chemistry;
- 3625 Descriptive mineralogy;
- 3924 High-pressure behavior