Eltyubyuite, Ca_(12)Fe_(10)~3+Si _4O_(32)Cl_6 - the Fe~3+ analogue of wadalite: A new mineral from the Northern Caucasus, Kabardino-Balkaria, Russia

摘要

Eltyubyuite (IMA2011-022), ideally Ca12Fe~3+10Si4O32Cl6 i.e. the Fe~3+ 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 mm 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. 800C, 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), Fe~2O3, Cl 9.60(0.48); O = Cl -2.13, Sum 98.26, and an empirical formula based on 26 cations, Ca12.12Mg0.04Ti0.11Fe9.41Al1.26Si2.98O31.89Cl5.04, which simplifies to Ca12(Fe~3+,Al)11Si3O32Cl5. Electron-back-scattered diffraction yields isometric symmetry, space group I43d (no. 220), a = 12.20(3), V = 1815.85(9) 3, 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- [Fe~3+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.