Abstract

Eltyubyuite, ideally Ca12Fe3+10Si4O32Cl6, a member of the mayenite supergroup, was originally described from altered xenoliths of the Upper Chegem, northern Caucasus, Russia, and Eifel, Germany, where it forms a solid-solution with wadalite (Ca12Al10Si4O32Cl6). The structure of the holotype was confirmed earlier using electron backscatter diffraction. The larger crystal size of Al-free eltyubyuite from a new occurrence in an altered carbonate–silicate xenolith enclosed in plagiodacites of the Shadil-Khokh volcano, Kel’ Plateau, Southern Ossetia, enabled the first direct refinement of the eltyubyuite crystal structure. At this locality, Al-free eltyubyuite occurs in a contact zone of the xenolith, within small veins composed of rusinovite, cuspidine and rondorfite. The structure of the Al-free eltyubyuite crystal (dimensions: 20 × 15 × 10 μm) was refined from X-ray diffraction data to R1 = 0.019. Eltyubyite (cubic, space group I4¯3d, a = 12.2150 (2) Å, V = 1822.55(6) Å3, Z = 2) is isostructural with mayenite. Both tetrahedral are Fe3+-dominant: the T1 site (= 1.848 Å) contains 0.85 Fe3+ and 0.15 Si4+, whereas the T2 site (= 1.766 Å) has 0.59 Fe3+ and 0.41 Si4+. Based on electron microprobe data, the empirical formula of eltyubyuite from Ossetia is Ca12.044(Fe3+10.373Si3.473Ti4+0.067Mn2+0.021Mg0.021)∑13.956O32Cl5.455. Raman spectroscopy recorded bands with increased half-width due to Fe3+ and Si4+ disorder at the two tetrahedral sites T1 and T2. The Raman bands at 959 and 901 cm−1 have been assigned to Si–O stretching vibrations (v1 and v3) of (SiO4)4−. The group of bands at 783 (v3), 705 (v1), 450 (v4), 307 (v2) cm−1 correspond to Fe–O vibration of (Fe3+O4)5−.

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