• journal_title：European Journal of Mineralogy
• Contributor：Evgeny V. Galuskin ; Irina O. Galuskina ; Radu Bailau ; Krystian Prusik ; Viktor M. Gazeev ; Aleksandr E. Zadov ; Nikolai N. Pertsev ; Lidia Jeżak ; Anatoly G. Gurbanov ; Leonid Dubrovinsky
• Publisher：E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
• Date：2013-02-01
• Format：text/html
• Language：en
• Identifier：10.1127/0935-1221/2013/0025-2285
• journal_abbrev：Eur J Mineral
• issn：0935-1221
• volume：25
• issue：2
• firstpage：221
• section：Articles

Eltyubyuite (IMA2011-022), ideally Ca₁₂Fe³⁺₁₀Si₄O₃₂Cl₆i.e. the Fe³⁺ analogue of wadalite, Ca₁₂Al₁₀Si₄O₃₂Cl₆, was discovered in altered silicate-carbonate xenoliths in the diatreme facies of ignimbrites in the Upper Chegem caldera, Kabardino-Balkaria, Northern Caucasus, Russia. Eltyubyuite forms light-brown or yellow crystals with tetrahedral habit up to 10 μm across in rondorfite or larnite grains and commonly overgrows wadalite. Associated minerals are hydroxylellestadite, edgrewite-hydroxyledgrewite, chegemite-fluorchegemite, cuspidine, lakargiite, perovskite, kerimasite, srebrodolskite and dovyrenite. Eltyubyuite formed by contact metamorphism of calcareous sediments under sanidinite-facies conditions (T > 800°C, P <1–2 kbar). Electron microprobe analysis (mean of 9 points) gave in weight% (s.d.): SiO₂ 9.57(0.32), TiO₂ 0.48(0.27), Al₂O₃ 3.45(1.81), MgO 0.08(0.07), CaO 36.84(0.91), Fe₂O₃, Cl 9.60(0.48); O = Cl −2.13, Sum 98.26, and an empirical formula based on 26 cations, Ca_12.12Mg_0.04Ti_0.11Fe_9.41Al_1.26Si_2.98O_31.89Cl_5.04, which simplifies to Ca₁₂(Fe³⁺, Al)₁₁Si₃O₃₂Cl₅. Electron-back-scattered diffraction yields isometric symmetry, space group I4̄d (no. 220), a = 12.20(3) Å, V = 1815.85(9) ų, Z = 2. Calculated density and refractive index are 3.349 g/cm³ and 1.85, respectively. The main bands in Raman spectra of eltyubyuite are attributed to [Fe³⁺O₄]⁵⁻: 700–710 cm⁻¹ (stretching vibrations), 460–470 cm⁻¹ (bending vibrations), whereas bands <400 cm⁻¹ are assigned to Ca-O and Ca-[Fe³⁺O₄]⁵⁻ vibrations. The mineral is named for the Balkarian village Eltyubyu, which is situated near the type locality. Eltyubyuite has subsequently been found in altered xenoliths within volcanic rocks of Eifel, Germany and Kel’ Highland (volcano Shadil-Khokh), Southern Ossetia.