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

Abstract

Eltyubyuite, ideally Ca 12 Fe 3+ 10 Si 4 O 32 Cl 6 , 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 (Ca 12 Al 10 Si 4 O 32 Cl 6 ). 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 I 4 ¯ 3 d , a = 12.2150 (2) A, V = 1822.55(6) A 3 , Z = 2) is isostructural with mayenite. Both tetrahedral are Fe 3+ -dominant: the T 1 site (= 1.848 A) contains 0.85 Fe 3+ and 0.15 Si 4+ , whereas the T 2 site (= 1.766 A) has 0.59 Fe 3+ and 0.41 Si 4+ . Based on electron microprobe data, the empirical formula of eltyubyuite from Ossetia is Ca 12.044 (Fe 3+ 10.373 Si 3.473 Ti 4+ 0.067 Mn 2+ 0.021 Mg 0.021 ) ∑13.956 O 32 Cl 5.455 . Raman spectroscopy recorded bands with increased half-width due to Fe 3+ and Si 4+ disorder at the two tetrahedral sites T 1 and T 2. The Raman bands at 959 and 901 cm −1 have been assigned to Si–O stretching vibrations ( v 1 and v 3 ) of (SiO 4 ) 4− . The group of bands at 783 ( v 3 ), 705 ( v 1 ), 450 ( v 4 ), 307 ( v 2 ) cm −1 correspond to Fe–O vibration of (Fe 3+ O 4 ) 5− .