CRYSTAL STRUCTURE OF "BETALOMONOSOVITE": NEW DATA
Crystal structure of mineral «betalomonosovite», {Ti 3.63Nb 0.03Fe 3+ 0.34Na 1.42Ca 0.71(O,OH,F) 4[Si 2O 7] 2} · {Na 3.00Mn 2+ 0.11[H 2PO 4] 2} from Rasvumchorr apatite deposits (Khibiny alkaline massif, Kola Peninsula) has been improved ( R = 0.056) within frame of the space group P1. The triclinic unit cell parameters are: a = 5.3184 Å, b = 7.0869 Å, c = 14.449 Å, α = 102.944°, β = 96.367°, γ = 90.331°. The microprobe analysis and X-ray diffraction study show that «betalomonosovite» contains more of titanium and lesser of sodium atoms as compared with the unit cell content of the mere lomonsovite. Numerous vacancies have been found in positions occupied mainly by sodium atoms. In highly defective sodium-phosphate modules, the position of phosphorus atom is split into two with equal probabilities, and two from four oxygen vertices of each phosphate tetrahedron are donors of hydrogen bonds (hydroxyl groups). The hypothesis of the «betalomonosovite» formed as metasomatic mineral is confirmed by peculiarities of its crystal structure which is stabilized by entropy as a result of the environment chemistry change due to creation of a large number of different type defects.