HIGH-PRESSURE BEHAVIOR, TRANSFORMATION AND CRYSTAL STRUCTURE OF THE SYNTHETIC IRON-FREE PIGEONITE

A single-crystal high pressure X-ray investigation was performed up to P = 6.5 GPa on a synthetic clinopyroxene of composition Ca0.15(1)Mg1.85(1)Si2.00(1)O6 [Di(15)En(85), unit-cell parameters at room pressure: a = 9.6525(6)Angstrom, b = 8.8461(2)Angstrom, c = 5.2036(5)Angstrom, beta = 108.370(5)degrees, V = 421.68(4)Angstrom(3)]. A first order P2(1)/c-C2/c displacive phase transition was found at P = 5.1 GPa; the transition was revealed by the disappearance of the b reflections (h + k = odd) and by sharp changes in the unit-cell parameters. Reversals through the transformation show that, if present, hysteresis is smaller than 0.1 GPa. The volume variation has been described by a third-order Birch-Mumaghan equation of state with V-0 = 421.68(8) Angstrom(3), K-T0 = 102(2) GPa, and K' = 8(1) for the low-symmetry phase (P2(1)/c) and with V-0 = 411.06(3) Angstrom(3) and K-T0 = 108(2) GPa for the high-symmetry phase (C2/c), with K' fixed to the value obtained for the low-symmetry form. The axial compressibility shows the following scheme: beta(b) > beta(A) = beta(c) > beta(asinbeta) for both phases. In comparison with pure clinoenstatite, Di(15)En(85) shows a similar step in unit-cell parameters at the transition, the disappearence of hysteresis and a decrease of transition pressure and of bulk modulus. Full intensity data sets were collected at room pressure, 2.6 and 4.5 GPa for the P2(1)/c phase and at P = 6.2 GPa for the C2/c phase. A slight increase of the intensity of h + k odd reflections and of the difference in the A and B chain kinking angles were observed. A comparison of the structural behavior of the P2(1)/c phase at high temperature and high pressure shows opposite behavior for M2-O bond lengths and O3-O3-O3 kinking angle.