Abstract
An unusually Mn-rich ilvaite sample from the Perda Niedda mine in Sardinia, Italy, was studied in order to clarify the Mn 2 + distribution among the different structural sites, and to observe the structural response of the mineral upon thermally induced oxidation-dehydrogenation. The crystal structure and the chemical composition of one crystal [a = 13.014(5), b = 8.867(3), c = 5.838(4) A, β = 90.02(4)°] were investigated. X-ray crystal-structure refinement, performed in the Pnam space group, and electron microprobe analyses yielded the formula (Ca 0 . 9 8 Mn 2 + 0 . 0 2 )(Fe 3 + Fe 2 + )(Mn 0 . 7 2 Fe 2 + 0 . 2 8 ) (Si 2 O 2 )O(OH). Crystal chemical details, compared to structural data from literature, led to the assumption that Mn 2 + replaces Fe 2 + , mainly at the M2 site. Annealing experiments and structure refinements were performed in the temperature range 400-690 °C. No phase transition was observed over the entire temperature range. Oxidation of Fe 2 + at the M1 site, with concomitant dehydrogenation, was deduced from examination of the structural adjustments occurring as the temperature was increased. A useful model to evaluate a possible OH - ↔ O 2 - substitution in ilvaite was obtained.
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