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Science 22 September 1978:
Vol. 201. no. 4361, pp. 1122 - 1123
DOI: 10.1126/science.201.4361.1122
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Articles

Crystal Chemistry of Silicon—Oxygen Bonds at High Pressure: Implications for the Earth's Mantle Mineralogy

R. M. HAZEN 1 and L. W. FINGER 1

1 Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20008

Transformations involving a change from tetrahedrally coordinated to octahedrally coordinated silicon (IVSi rarr VISi) are observed to occur at high pressure when the mean IVSi-O bond compresses to approximately 1.59 angstroms based on known room-pressure crystal structures, Si-O bond compressibilities, and pressures of IVSi rarr VISi transformations. The lower two-thirds of the mantle transition zone of high-density gradient (500 to 900 kilometers) corresponds to the predicted range of IVSi rarr VISi transformations. The 10 percent density increase of this zone at zero pressure is attributed primarily to the density increase associated with the change in silicon coordination. Below 900 kilometers all silicon is predicted to be in octahedral or greater coordination. The concept of cation polyhedral stability fields is defined.

Submitted on February 27, 1978
Revised on July 18, 1978

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Science. ISSN 0036-8075 (print), 1095-9203 (online)