• journal_title：American Mineralogist
• Contributor：Yassir A. Abdu ; Frank C. Hawthorne
• Publisher：Mineralogical Society of America
• Date：2013-07-01
• Format：text/html
• Language：en
• Identifier：10.2138/am.2013.4328
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：98
• issue：7
• firstpage：1227
• section：Articles

Three synthetic Ca-rich clinopyroxenes on the hedenbergite-ferrosilite join, with compositions (Hd₅₀Fs₅₀), (Hd₆₀Fs₄₀), and (Hd₇₀Fs₃₀), were investigated by single-crystal X-ray diffraction and Mössbauer 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 Fe²⁺, respectively. The Mössbauer spectra were analyzed by a quadrupole splitting distribution (QSD) method, and the hyperfine parameters of the QSD fit indicate the presence of Fe²⁺ in three local environments that are similar to those of Fe²⁺ 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 Mössbauer spectra of Ca-rich C2/c clinopyroxenes.