Pressure-induced dehydration of dioptase: A single-crystal X-ray diffraction and Raman spectroscopy study - 19/11/18

Doi : 10.1016/j.crte.2018.07.007

Fei Qin ^a,^b,^{^{1
Present address: School of Earth Sciences, University of Bristol, Queens Road, Bristol, UK.}}, Xiang Wu ^c, Shan Qin ^a,^⁎ , Dongzhou Zhang ^d, Vatali B. Prakapenka ^e, Steven D. Jacobsen ^b
^a Key Laboratory of Orogenic Belts and Crustal Evolution, MOE, Peking University and School of Earth and Space Sciences, Peking University, Beijing, China
^b Department of Earth and Planetary Sciences, Northwestern University, Evanston, IL, USA
^c State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, China
^d School of Ocean and Earth Science and Technology, Hawai’i Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, Hawaii, USA
^e Center for Advanced Radiation Sources, University of Chicago, Chicago, IL, USA

^⁎Corresponding author.

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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 β_a>β_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.