The effect of iron on the P2 (sub 1) /c to C2/c transition in (Mg,Fe)SiO (sub 3) clinopyroxenes

High-P Raman spectroscopy has been used to study the effect of iron on the low-clinoenstatite (P2 (sub 1) /c) to high-density clinoenstatite (C2/c) transition for clinopyroxenes along the MgSiO (sub 3) -FeSiO (sub 3) join. For the MgSiO (sub 3) end-member, with rising P the transition to the C2/c structure occurs at 7.89 GPa and reverts to P2 (sub 1) /c symmetry with decreasing P. Both the transition P and the hysteresis of the transition decreases markedly with addition of iron; the transition has been bracketed to + or - 0.5 GPa for compositions with X (sub Fe) > or = 30 %. The variation of the transition P with increasing Fe can be described in the equation: P (sub t) (GPa) = (5.97 + or - 0.14) - (4.26 + or - 0.23)X (sub Fe) . The decrease in transition P and the hysteresis of the P2 (sub 1) /c to C2/c transition observed in Fe-bearing clinopyroxenes may be due, in part, to crystal field stabilization energy arising from iron in the octahedral sites of the C2/c structure.