Mayenite supergroup, part IV: Crystal structure and Raman investigation of Al-free eltyubyuite from the Shadil-Khokh volcano, Kel’ Plateau, Southern Ossetia, Russia

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 I$4¯$3d, a = 12.2150 (2) Å, V = 1822.55(6) ų, 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 Ca_12.044(Fe³⁺_10.373Si_3.473Ti⁴⁺_0.067Mn²⁺_0.021Mg_0.021)_∑13.956O₃₂Cl_5.455. 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⁻.