Batisite, Na₂BaTi₂(Si₄O₁₂)O₂, from Inagli massif, Aldan, Russia: crystal-structure refinement and high-temperature X-ray diffraction study

Abstract

The crystal structure of batisite, Na₂BaTi₂ (Si₄O₁₂)O₂, from the Inagli massif (Aldan, Yakutia, Russia) was refined to R ₁ = 0.032 for 1449 unique observed reflections. The mineral is orthorhombic, Imma, a = 8.0921(5), b = 10.4751(7), c = 13.9054(9) Å, V = 1178.70(13) ų. The mineral is based upon three-dimensional titanosilicate framework consisting of chains of corner-sharing MO₆ octahedra (M = Ti, Nb, Fe and Zr) and vierer chains of corner-sharing SiO₄ tetrahedra. Both chains are parallel to the a axis and are linked by sharing peripheral O atoms. The octahedral chains display disorder of M atoms and bridging O sites related to the out-of-center distortion of octahedral geometry around Ti⁴⁺ cations. Electron microprobe analysis gives SiO₂ 39.46, TiO₂ 24.66, BaO 21.64, Na₂O 7.56, K₂O 4.38, Fe₂O₃ 0.90, ZrO₂ 0.66, Nb₂O₅ 0.36, (H₂O)_calc 0.58, sum 99.76 wt%. The seven strongest X-ray powder-diffraction lines [listed as d in Å (I) hkl] are: 8.39 (94) 011, 3.386 (56) 031, 3.191 (36) 123, 2.910 (46) 222, 2.896 (100) 024, 2.175 (45) 035, 1.673 (57) 055. The thermal behaviour of batisite in the temperature range from 25 to 950 °C was studied using high-temperature powder X-ray diffraction. The thermal expansion coefficients along the principal crystallographic axes are: α _a  = 14.4 × 10⁻⁶, α _b  = 8.7 × 10⁻⁶, α _c  = 8.4 × 10⁻⁶, α _V  = 31.5 °C⁻¹ for the temperature range 25–500 °C and α _a  = 19.6 × 10⁻⁶, α _b  = 9.1 × 10⁻⁶, α _c  = 8.8 × 10⁻⁶, α _V  = 37.6 °C⁻¹ for the temperature range 500–900 °C. The direction of maximal thermal expansion is parallel to the chains of both MO₆ octahedra and SiO₄ tetrahedra, which can be explained by the stretching of silicate chains due to the increasing thermal vibrations of the Ba²⁺ cations. At 1000 °C, the titanosilicate framework in batisite collapses with the formation of fresnoite, Ba₂TiSi₂O₇O.