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

Transformations involving a change from tetrahedrally coordinated to octahedrally coordinated silicon ((IV)Si --> (VI)Si) are observed to occur at high pressure when the mean (IV)Si-O bond compresses to approximately 1.59 angstroms based on known room-pressure crystal structures, Si-O bond compressibilities, and pressures of (IV)Si --> (VI)Si transformations. The lower two-thirds of the mantle transition zone of high-density gradient (500 to 900 kilometers) corresponds to the predicted range of (IV)Si --> (VI)Si 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.