A new phase of coesite SiO2

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Abstract
A coesite crystal synthesized from aqueous solution (Arndt and Rombach, 1976) under static high-pressure high-temperature conditions of 44 – 49 kb and 610°C was investigated by X-ray diffraction. From the lattice constants a = 7.148(2)Å, b = 12.334(3)Å, c = 7.112(2)Å, β = 120.30(2)°, and the space group P21/a we conclude the existence of another coesite phase, which is about 1 % denser than the known phase with C2/c (Gibbs et al., 1977). The overall arrangement of the atoms in the new phase is closely related to the C2/c phase, but the SiO4 tetrahedra are of considerably different sizes. The finding of a new, denser coesite phase implies that the silica phase diagram may consist of even more phases than presently known. The range of bond lengths and bond angles was used to check for linear correlations which would give supporting experimental evidence for the 7r-bond model in the silicate ion (Cruickshank, 1961).

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