The effect of iron on the P2(1)/c to C2/c transition in (Mg,Fe)SiO3 clinopyroxenes

High-pressure Raman spectroscopy has been used to investigate the effect of iron on the low-clinoenstatite (space group P2(1)/c) to high-density clino enstatite (space-group C2/c) transition for clinopyroxenes along the MgSiO3 -FeSiO3 join. For the MgSiO3 endmember, the transition to the C2/c structur e occurs at 7.89 GPa upon increasing pressure and reverts to P2(1)/c symmet ry at 4.59 GPa upon decreasing pressure. Both the transition pressure and t he hysteresis of the transition decreases markedly with addition of iron. T he transition has been bracketed between 0.5 GPa or less for compositions w ith X-Fe greater than or equal to 30 %. The variation of the transition pre ssure with increasing iron content can be described by the following equati on: p(t) (GPa) = (5.97 +/- 0.14) - (4.26 +/- 0.23) X-Fe. The decrease in transition pressure and the hysteresis of the P2(1)/c to C- 2/c transition observed in Fe-bearing clinopyroxenes may be due, in part, t o crystal field stabilization energy arising from iron in the octahedral si tes of the C-2/c structure.