Pressure-induced dehydration of dioptase: A single-crystal X-ray diffraction and Raman spectroscopy study

摘要

We present a synchrotron-based, single-crystal X-ray diffraction and Raman spectroscopy study of natural green dioptase (Cu6Si6O18 center dot 6H(2)O) up to similar to 30 GPa at room temperature. The lattice parameters of dioptase exhibit continuous compression behavior up to similar to 14.5 GPa, whereupon a structural transition is observed. Pressure-volume data below 14.5 GPa were fitted to a second-order Birch-Murnaghan equation of state with V-0 = 1440(2) angstrom(3) and K-0 = 107(2) GPa, with K-0(') = 4(fixed). The low-pressure form of dioptase exhibits anisotropic compression with axial compressibility beta(a) beta(c) in a ratio of 1.14:1.00. Based on the diffraction data and Raman spectroscopy, the new high-pressure phase could be regarded as a dehydrated form of dioptase in the same symmetry group. Pressure-induced dehydration of dioptase contributes broadly to our understanding of the high-pressure crystal chemistry of hydrous silicates containing molecular water groups. (C) 2018 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.