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)°, 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)∑2Na1(Rb0.06Cs0.020.92)∑1(Fe2+9.11Mg0.46Mn0.22 Al0.10Na0.09Ca0.02)∑10 (Ti1.90Nb0.05Mg0.03Zr0.02)∑2[(Si11.81Al0.19)∑12O34]O2(OH)6 (F0.86O0.14)∑1(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)O34apfu, ideally (Si6O17)2pfu. 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)1.390.61] 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)2pfu. Nafertisite is closely related to the astrophyllite-group minerals.

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