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A single-crystal high pressure X-ray investigation was performed up to P = 6.5 GPa on a synthetic clinopyroxene of composition Ca 0.15 (1)Mg 1.85 (1)Si 2.00 (1)O 6  [Di 15 En 85 , unit-cell parameters at room pressure: a = 9.6525(6)Å, b = 8.8461(2)Å, c = 5.2036(5)Å, β = 108.370(5)°, V = 421.68(4)Å 3 ]. A first order P21/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-Murnaghan equation of state with V 0  = 421.68(8) Å 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) Å 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: β b  &gt; β a  ≅ β c  &gt; β asinβ  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.

High pressure behavior, transformation and crystal structure of synthetic iron-free pigeonite