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

Abstract

We present a synchrotron-based, single-crystal X-ray diffraction and Raman spectroscopy study of natural green dioptase (Cu6Si6O18·6H2O) up to ∼30GPa at room temperature. The lattice parameters of dioptase exhibit continuous compression behavior up to ∼14.5GPa, whereupon a structural transition is observed. Pressure–volume data below 14.5GPa were fitted to a second-order Birch–Murnaghan equation of state with V0=1440(2) Å3 and K0=107(2) GPa, with K0′=4(fixed). The low-pressure form of dioptase exhibits anisotropic compression with axial compressibility βa>βc 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.