Local structure in C2/c clinopyroxenes on the hedenbergite (CaFeSi _2O_6)-ferrosilite (Fe_2Si_2O _6) join: A new interpretation for the M?ssbauer spectra of Ca-rich C2/c clinopyroxenes and implications for pyroxene exsolution

摘要

Three synthetic Ca-rich clinopyroxenes on the hedenbergite-ferrosilite join, with compositions (Hd_(50)Fs_(50)), (Hd _(60)Fs_(40)), and (Hd_(70)Fs_(30)), were investigated by single-crystal X-ray diffraction and M?ssbauer spectroscopy at room temperature. For all samples, there is no evidence for the presence of phases with symmetry other than C2/c. Structure refinement, using a split-atom model, shows the existence of two different local configurations in the M2 cavity, a hedenbergite-like arrangement and a C2/c clinoferrosilite-like arrangement that are centered by Ca and Fe~(2+), respectively. The M?ssbauer spectra were analyzed by a quadrupole splitting distribution (QSD) method, and the hyperfine parameters of the QSD fit indicate the presence of Fe~(2+) in three local environments that are similar to those of Fe~(2+) at the M1 site in hedenbergite, and the M1 and M2 sites in C2/c clinoferrosilite, in agreement with the structure-refinement results. Our results are consistent with the calculated phase diagram for hedenbergite-ferrosilite, where a miscibility gap and a two-phase field of metastable hedenbergite and metastable pigeonite occur at low temperature. The coexistence of a hedenbergite-like configuration around Ca and a C2/c clinoferrosilite-like configuration around Fe can be considered as a precursor for pyroxene exsolution at low temperature. Our results support previous structure-refinement work on Ca-rich clinopyroxenes along the diopside-enstatite join, and provide a new insight in the interpretation of M?ssbauer spectra of Ca-rich C2/c clinopyroxenes.