COMPRESSION MECHANISMS OF COESITE

Abstract

The structure of coesite has been determined at ten pressures up to a maximum of 8.68 GPa by single-crystal X-ray diffraction. The dominant mechanism of compression is the reduction of four of the five independent Si-O-Si angles within the structure. There is no evidence of the fifth linkage, Si1-O1-Si1, deviating from 180°. Some Si-O bond distances also decrease by up to 1.6% over the pressure range studied. The pattern of Si-O-Si angle reduction amounts to a rotation of the Si2 tetrahedron around the [001] direction. This rotation induces significant internal deformation of the Si1 tetrahedron. Comparison of the experimental data with rigid-unit distance least-squares simulations of coesite suggests that this pattern of compression, the anomalous positive values of both s23 and K'' in the equation of state of coesite, its high elastic anisotropy and the unusual straight Si1-O1-Si1 linkage within the structure are all consequences of the connectivity of the tetrahedral framework.