Nafertisite, Na3Fe2+10Ti2(Si6O17)2O2(OH)6F(H2O)2, from Mt. Kukisvumchorr, Khibiny alkaline massif, Kola peninsula, Russia:

Abstract

The crystal structure of nafertisite, Na 3 Fe 2+ 10 Ti 2 (Si 6 O 17 ) 2 O 2 (OH) 6 F(H 2 O) 2 , from Mt. Kukisvumchorr, Khibiny alkaline massif, Kola peninsula, Russia, a 5.358(1), b 16.204(3), c 21.976(4) A, β 94.91(1) ° , V 1901.0(7) A 3 , space group A 2/ m , Z = 2, D calc. 3.116 g/cm 3 , has been refined to an R 1 value of 5.60 % for 2747 observed [ F o > 4σ F ] 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 Fe 3+ /(Fe 2+ +Fe 3+ ) ratio was measured by Mossbauer spectroscopy, and FTIR and Raman spectra were collected. The empirical formula was calculated on the basis of (O + F + OH + H 2 O) = 44.39 pfu : (Na 1.39 K 0.61 ) ∑2 Na 1 (Rb 0.06 Cs 0.02 □ 0.92 ) ∑1 (Fe 2+ 9.11 Mg 0.46 Mn 0.22 Al 0.10 Na 0.09 Ca 0.02 ) ∑10 (Ti 1.90 Nb 0.05 Mg 0.03 Zr 0.02 ) ∑2 [(Si 11.81 Al 0.19 ) ∑12 O 34 ]O 2 (OH) 6 (F 0.86 O 0.14 ) ∑1 (H 2 O) 1.39 ; Z = 2; the content of H 2 O 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 Fe 2+ -dominant M(1–3) octahedra, the M (1–3) sites are fully occupied and the ideal composition of the O sheet is Fe 2+ 10 apfu . The H sheet is composed of the nafertisite T 6 O 17 ribbons and Ti-dominant D octahedra. The chemical composition of the nafertisite ribbon is (Si 11.81 Al 0.19 )O 34 apfu , ideally (Si 6 O 17 ) 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 (Na 1.39 K 0.61 ), Na, (□ 0.92 Rb 0.06 Cs 0.02 ) and [(H 2 O) 1.39 □ 0.61 ] pfu , respectively. The chemical composition of the I block of the general formula A 2 BCW is [(Na 1.39 K 0.61 )Na 1 (□ 0.92 Rb 0.06 Cs 0.02 )(H 2 O) 1.39 ] pfu , ideally Na 3 (H 2 O) 2 pfu. Nafertisite is closely related to the astrophyllite-group minerals.