Accurate structure geometry of P2/n omphacite at variable pressures

Our recent high-pressure (HP) Raman spectroscopy study of natural omphacite (space group P2/n, with composition close to Jd45Di55 from Münchberg Massif, Germany) up to 10 GPa revealed some interesting changes in elastic behavior. The pressure dependence of the wavenumber of few Raman peaks shows a discontinuity near 5 GPa, that can be related to the homogenization of the volume compressibility associated with chemical order at the six- and eight-coordinated sites in the structure. Available HP diffraction data of omphacites [1-3] is not sufficient to make a one-to-one comparison with the pressure dependence of the structure and our HP Raman results. Considering this we have recently done a HP single-crystal diffraction study of the same sample at the Xpress beamline of the Elettra Synchrotron, Trieste. The crystal, previously cut in two different perpendicular directions with a focused ion beam, was loaded to the diamond anvil cell (DAC) to maximize the available reciprocal space information during our HP investigation. This permitted us to overcome the limitations in accessible reciprocal space in usual HP studies and enabled us to obtain a structure refinement with anisotropic atomic displacement parameters (adp’s). In particular, the data was sufficient to refine adp’s of all the atoms in the structure of omphacite while changing pressure, and to obtain accurate structural variations in terms of deformation of bonds and coordination polyhedra. The description of the elastic and structural behavior of omphacite crystals during compression will further help to understand the genetic processes of omphacite-rich rocks, such as eclogites. References: [1] McCormick et al. (1989) Am Mineral 74:1287-1292 [2] Pandolfo et al. (2012) Am Mineral 97:407-414 [3] Zang et al. (2016) Phys Chem Miner 43:707-715