Cation distribution and local crystal chemistries of calcium-containing pyroxenes

J. M. Amigó, Billy J. EVANS, Stefan HAFNER


Irregularities have been observed in the Curie temperatures of a series of synthetic pyroxenes, CaxFel-,SiO,. For example, the x = 0.15 sample exhibited a Curie point of 40K, whereas for x = 0.02 and x = 0.10, the Curie points were 40K and 37K, respectively. These results suggest that one or more of these samples is not single phase or deviates from the reported stoichiometry. The 3OOK Mossbauer spectra of these samples have been reported to lead to erroneous cation distributions. The magnetic properties suggest that the cation distributions might be in error because of deviations o£ the sample composition from the expected ones. Since a considerable body of thermodynamic and crystal chemistry depends on a clarification of the basis for the apparently erroneous cation distributions in cakium-containing clinopyroxenes, we have reinvestigated the "Fe Mossbauer spectra of the synthetic pyroxenes at 300K and between 60K and 4.2K. Our results are in qualitative agreement with those reported earlier on these specimens, but absorption features have been observed near 0.8 mm S-' which have not been reported before. Impurity Fe-containing phases or non-equilibrium single-phase materials are implicated as the sources of the absorptions near 0.8 mm S-' by the erratic magnetic properties of some of the sarnples and by the fact that even for dinopyroxenes, these samples exhibit spectra with unusual features.

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