Mayenite supergroup, part IV: Crystal structure and Raman investigation of Al-free eltyubyuite from the Shadil-Khokh volcano, Kel' Plateau, Southern Ossetia, Russia
Gfeller, Frank; Środek, Dorota; Kusz, Joachim; Dulski, Mateusz; Gazeev, Viktor; Galuskina, Irina; Galuskin, Evgeny; Armbruster, Thomas
European Journal of Mineralogy Volume 27 Number 1 (2015), p. 137 - 143
published: Feb 1, 2015
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 R 1 = 0.019. Eltyubyite (cubic, space group I43d, a = 12.2150 (2) Å, V = 1822.55(6) Å3, Z = 2) is isostructural with mayenite. Both tetrahedra 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)Σ13.956O32Cl5.455. 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 (ν1 and ν3) of (SiO4)4–. The group of bands at 783 (ν3), 705 (ν1), 450 (ν4), 307 (ν2) cm–1 correspond to Fe–O vibration of (Fe3+O4)5–.