A P2(1)/c-C2/c high-pressure phase transition in Ca0.5Mg1.5Si2O6 clinopyroxene

A high-pressure P2(1)/c-C2/c phase transition in a synthetic iron-free clin opyroxene of composition Ca0.5Mg1.5Si2O6 was observed at pressure between 3 and 5 Gpa from powder diffraction data collected up to P = 14.2 GPa in a d iamond anvil cell by means of synchrotron radiation. The transition is mark ed by a continuous decrease in a, c, and P cell parameters in the transitio n range and by the disappearance of reflections with h+k odd. No hysteresis could be found. The spontaneous strain due to the transition occurs almost completely on the (010) plane and is described by a strong compression at a direction of 150 degrees from the c axis and a milder expansion at 600 fr om the c axis. Interaction between the macroscopic cell strain and microsco pic strain due to compositional heterogenities may explain the difference f rom the transition behavior in clinoenstatite. A third-order Birch-Murnagha n equation of state for the C2/c high-pressure phase was refined giving the following parameters: V-0 = 429(2) Angstrom (3), K = 99(7) GPa, K ' = 6.5 (w chi (2) = 1.3). Only minor differences are observed with other iron-free clinopyroxenes. The compressional strain in the C2/c phase in the Ca0.5Mg1 .5Si2O6 pyroxene has almost the same orientation as in diopside and in Ca0. 8Mg1.2Si2O6 pyroxene, displaying higher compression on (010) at 1400 from t he c axis and suggesting a similar compressional mechanism for Ca-rich C2/c clinopyroxenes.