INTERSITE PARTITIONING OF MG AND FE IN CA-FREE HIGH-PRESSURE C2 C CLINOPYROXENE/

The intracrystalline distribution of Mg and Fe2+ on the M1 and M2 octa hedral sites in high-P C2/c (Mg,Fe)SiO3 clinopyroxene has been determi ned on quenched samples using Fe-57 Mossbauer spectroscopy. Although t he recovered samples that were measured had the low-P (P2(1)/c) struct ure, the ordering state of the high-P polymorph is believed to have be en preserved because the high-P C2/c --> low-P P2(1)/c transformation occurred at room temperature during the final stages of decompression of the experiments. Under such conditions significant cation diffusion is unlikely. Low temperature (81 K) Mossbauer spectra indicate that F e2+ is strongly ordered onto the M2 sites, with an average K-D Angstro m 4.0(8) for the intersite exchange reaction: Fe2+(M1) + Mg2+(M2) = Fe 2+(M2) + Mg2+(M1) (1) Because the samples were quenched from high temp erature (1200-1275 degrees C) at 9.5 GPa, the temperature to which the measured ordering state corresponds remains problematic. However, a s ample re-annealed at 1000 degrees C is more ordered than the original sample synthesized at 1275 degrees C, suggesting that high-temperature cation distributions in high-P clinopyroxene can be quenched in the m ulti-anvil experiments. Therefore, we conclude that the measured catio n distributions correspond approximately to the conditions of the expe riments. A small compositional dependence of cation ordering also exis ts, indicating subregular behavior across the binary join. A fit to th e data yields Delta G(exch)degrees = -11.9 (2.0) kJ/mol at 1275 degree s C for the exchange Reaction 1, which describes the ordering behavior on the octahedral sites in the clinopyroxenes. Comparison with data f or orthopyroxene, which were obtained mostly at 1 atm, indicates that the state of ordering in orthopyroxene and high-P clinopyroxene is sim ilar, especially if cation ordering in the high-P clinopyroxene has a small pressure dependence. The similarities are consistent with the ob served structural similarities between the two polymorphs.