Кристаллохимия высоконатриевого чильманита-(Cе) (Хибинский массив, Кольский полуостров) // ЗРМО.

Abstract

Chemical composition, crystal structure, and Raman spectrum have been investigated for the Na-rich kihlmanite-(Ce). Chemical composition of this mineral: Na2Î 4.83, SiO2 9.76, CaO 0.94, TiO2 11.74, Nb2O5 1.83, La2O3 10.63, Ce2O3 26.22, Pr2O3 2.23, Nd2O3 9.00, Sm2O3 1.30, CO2 14.40, H2O 6.00, corresponds to empirical formula: (Na0.95Ca0.11)1.06(Ce0.98La0.40Nd0.33Pr0.08Sm0.05)1.85

(Ti0.91Nb0.08)0.99O2[Si1.00O4] {(HCO3)1.24(ÑÎ3)0.76}2 Í2O. Raman spectrum contains following lines: 850, 722, (n3, Si—O) 564, 610, 424 (n2, n4, Si-O), 1069, 919, 964 (n1, CO3, HCO3), 675 (n4, CO3), 388 (CaO6, NaO6), 290 (TiO6) cm–1. Crystal structure is refined in the space group P1 (a = 5.0005(3), b = = 7.5273(6), ñ = 15.3344(16) Å, a = 101.950(8), b = 92.938(7), g = 109.323(7)°) up to the value of crystallographic repeatability factor R1 = 0.054 for 1842 independent reflections (|Fo| > 4sF). Obtained crystal-chemical formula may be written as following: (Na1.10Ca0.11)1.21Ce2.00(Ti0.77Nb0.23)1.00

O2.00 [Si1.00O4.00]{(CO3)1.55(HCO3)0.45}2.00 H2O1.16. Na atoms occupy in the structure three independent positions: Na(l), Na(2) and Na(3). The presence of additional sodium site, relatively to the standard kihlmanite-(Ce), enforces chemical bonds between «tundrite» layers due to decrease of the unit cell parameter ñ by ~ 0.1 Å. Occurrence of sodium in the kihlmanite-(Ce) structure by the scheme: 9 + = + + HOC Na CO 3 – 3 2– leads to the dominance of CO3 2– groups in hydrocarbonate position. Key words: kihlmanite-(Ce), Na-rich, crystal structure, Raman spectrum, Khibiny massif.