Nafertisite, Na3Fe2+10Ti2(Si6O17)2O2(OH)6F(H2O)2, from Mt. Kukisvumchorr, khibiny alkaline massif, kola peninsula, Russia : refinement of the crystal structure and revision of the chemical formula

• Published in: European Journal of Mineralogy
• Main Authors: F. Cámara, E. Sokolova, Y. A. Abdu, F. C. Hawthorne
• Other Authors: F, ., C, á, m, a, r, ; E, S, o, k, l, v, ; Y, A, b, d, u, ; F, H, w, t, h, n, e
• Format: Article in Journal/Newspaper
• Language: English
• Published: Schweizerbart'sche 2014
• Subjects: Electron-microprobe analysi; FTIR spectroscopy; Mössbauer spectroscopy; Nafertisite; Nafertisite T6O17 ribbon; Raman spectroscopy; Structure refinement; Settore GEO/06 - Mineralogia; kola peninsula
• Online Access: http://hdl.handle.net/2434/477220; https://doi.org/10.1127/0935-1221/2014/0026-2401

The crystal structure of nafertisite, Na3Fe2+10Ti2(Si6O17)2O2(OH)6F(H2O)2, from Mt. Kukisvumchorr, Khibiny alkaline massif, Kola peninsula, Russia, a 5.358(1), b 16.204(3), c 21.976(4) Å , β 94.91(1)Å3, V 1901.0(7) Å3, space group A2/m, Z = 2, Dcalc. 3.116 g/cm3, has been refined to an R1 value of 5.60%for 2747 observed [Fo.4σF] unique reflections collected with a single-crystal diffractometer. The crystal used in the collection of the X-ray intensity data was analyzed by electron microprobe; the Fe3+/ (Fe2++Fe3+) ratio was measured by Mössbauer spectroscopy, and FTIR and Raman spectra were collected. The empirical formula was calculated on the basis of (O + F + OH + H2O) = 44.39 pfu: (Na1.39K0.61)σ2Na1(Rb0.06Cs0.02□0.92)σ1(Fe2+9.11Mg0.46Mn0.22 Al0.10Na0.09Ca0.02)σ10 (Ti1.90Nb0.05Mg0.03Zr0.02)σ2[(Si11.81Al0.19)sigma;12O34]O2(OH)6 (F0.86O0.14)σ1(H2O)1.39; Z=2; the content of H2O was calculated from the structure-refinement results. The HOH layer is the main structural unit in nafertisite. It consists of a central octahedral (O) sheet and two adjacent heteropolyhedral (H) sheets. The O sheet is composed of Fe2+-dominant M(1-3) octahedra, the M(1-3) sites are fully occupied and the ideal composition of the O sheet is Fe2+10apfu. The H sheet is composed of the nafertisite T6O17 ribbons and Ti-dominant D octahedra. The chemical composition of the nafertisite ribbon is (Si11.81Al0.19)O34 apfu, ideally (Si6O17)2 pfu. In the crystal structure of nafertisite, HOH layers alternate with intermediate (I) blocks along c. In the I block, the A, B, C and W sites are occupied by (Na1.39K0.61), Na, (□;0.92Rb0.06Cs0.02) and [(H2O)1.39□0.61] pfu, respectively. The chemical composition of the I block of the general formula A2BCW is [(Na1.39 K0.61)Na1(□0.92Rb0.06Cs0.02)(H2O)1.39] pfu, ideally Na3(H2O)2 pfu. Nafertisite is closely related to the astrophyllite-group minerals.