Mossbauer Spectroscopy and Crystal Chemistry of Aenigmatites

VIII VI IV Aenigmatite, Na4 (Fe2 +,Ti,Fe 3 +) 1 2 (SiFe 3 +) 12 0 40 , is a common constituent of sodium-rich alkaline igneous rocks and is classified as an open-branched single-chain silicate, whose structure is closely related to that of sapphirine. The first Mossbauer spectra of three valid aenigmatite specimens were recorded and the detailed crystal chemistry were obtained. In the Mossbauer spectra, the extreme overlap of peaks for Fe 2 + and Fe3+ cations in the low velocity region made fitting difficult and even made it impossible to resolve he liquid N2 spectra. Moreover, the Mossbauer spectra are complicated by the possible occurrence of iron cations in seven octahedral and four tetrahedral sites. However, the Fe2+-bearing octahedral sites were grouped into three categories, resulting in three quadrupole doublets. The Fe3+ cations in octahedral and tetrahedral sites were accounted for by three peaks. As a result, the stringent fitting of the room temperature spectra led to the resolution of nine peaks. Such peaks consist of three doublets of Fe2 +/oct and one combined peak at low velocity corresponding to two small peaks at high velocity which were assigned to Fe3+/tet and Fe3 +/oct. Using the peak areas for Fe2+ and Fe3+ peaks, analytical data were reevaluated, including the comparison of Fe3+/Fe2+ ratios determined by wet chemistry and by Mossbauer spectroscopy. As a result, electron microprobe