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

作者：Gfeller Frank^*; Srodek Dorota; Kusz Joachim; Dulski Mateusz; Gazeev Viktor; Galuskina Ireta; Galuskin Evgeny; Armbruster Thomas

来源：European Journal of Mineralogy, 2015, 27(1): 137-143.

DOI：10.1127/ejm/2015/0027-2421

• Eltyubyuite
• wadalite
• mayenite supergroup
• crystal structure
• Raman spectroscopy
• Shadil-Khokh volcano
• Southern Ossetia

Eltyubyuite, ideally Ca12Fe103+Si4O32Cl6, 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 ShadilKhokh 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 x 15 x 10 mu m) was refined from X-ray diffraction data to R-1 = 0.019. Eltyubyite (cubic, space group I (4) over bar 3d, a= 12.2150(2) angstrom, V= 1822.55(6) angstrom(3), Z = 2) is isostructural with marnite. Both tetrahedra are Fe3+-dominant: the T1 site (= 1.848 angstrom) contains 0.85 Fe3+ and 0.15 Si4+, whereas the T2 site (= 1.766 angstrom) has 0.59 Fe3+ and 0.41 Si4+. Based on electron microprobe data, the empirical formula of eltyubyuite from Ossetia is Ca-12.044(Fe10.3733+Si3.473Ti4+ (0.067) Mn0.0212+Mg0.021)(Sigma 13.956)O32Cl5.455. 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 (nu(1) and nu(3)) of (SiO4)(4-). The group of bands at 783 (nu(3)), 705 (nu(1)), 450 (nu(4)), 307 (nu(2)) cm(-1) correspond to Fe-O vibration of (Fe3+O4)(5-).

• 出版日期2015-2