Synchrotron X-ray powder diffraction study of natural P2/n-omphacites at high-pressure conditions

Synchrotron X-ray powder diffraction experiments at high pressure condition s (0.0001-13 GPa) were performed at ESRF (Grenoble-F). on the beamline ID9, to investigate the bulk elastic properties of natural P2/n-omphacites. wit h quasi-ideal composition. The monoclinic cell parameters a, b, c and beta were determined as a function of pressure, and their compressibility coeffi cients are 0.00277(7), 0.00313(8), 0.00292(5) and 0.00116(4) GPa(-1), respe ctively. The third-order Birch-Murnaghan equation of state was used to inte rpolate the experimental P - V data, obtaining K-0 = 116.6(+/-2.5) GPa and K-0' = 6.03(+/-0.60). K-0 was also determined by means of the axial and ang ular compressibilities [122.5(+/-1.7) GPa], and of the finite Lagrangian st rain theory [121.5(+/-1.0) GPa]. The discrepancies on K-0 are discussed in the light of a comparison between techniques to determine the bulk modulus of crystalline materials from static compression diffraction data.