Crystal chemistry of Na-rich kihlmanite-(Ce) from arfvedsonite-aegirine-microcline pegmatite at the Kihlman Mt. (Khibiny massif, Kola Peninsula, Russia)

Panikorovskii T. L., Kalashnikova G. O., Zhitova E. S., Pakhomovsky Ya. A., Bocharov V. N., Yakovenchuk V. N., Zolotarev A. A. jr., Krivovichev S. V.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2017. V. 146. N 2. P. 113-124

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Language: Russian

Abstract

Chemical composition, crystal structure, and Raman spectrum have been investigated for the Na-rich kihlmanite-(Ce). Chemical composition of this mineral: Na2O 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 (CO3)0.76}2 · H2O. Raman spectrum contains following lines: 850,722, (v3, Si‑O) 564, 610, 424 (v2, v4, Si‑O), 1069, 919, 964 (v1 CO3, HCO3), 675 (v4, CO3), 388 (CаO6, NaO6), 290 (TiO6) cm–1. Crystal structure is refined in the space group P1 (a = 5.0005(3), b = 7.5273(6), с = 15.3344(16) Å, α = 101.950(8), β = 92.938(7), γ = 109.323(7)°) up to the value of crystallographic repeatability factor R1 = 0.054 for 1842 independent reflections (|F0| > 4F). Obtained crystal-chemical formula may be written as following: (Na1.10Ca0.11)1.21Ce2.00(Ti0.77Nb0.23)1.00O2.00·[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 с by ~ 0.1 Å. Occurrence of sodium in the kihlmanite-(Ce) structure by the scheme: □ + - = Na+ +  – leads to the dominance of  groups in hydrocarbonate position.

Key words: kihlmanite-(Ce), Na-rich, crystal structure, Raman spectrum, Khibiny massif.