Nafertisite, Na_3Fe_10~(2+)Ti_2(Si_6O_(17))_2O_2(OH)_6F(H_2O)_2, from Mt. Kukisvumchorr, Khibiny alkaline massif, Kola peninsula, Russia: Refinement of the crystal structure and revision of the chemical formula

摘要

The crystal structure of nafertisite, Na_3Fe_10~(2+)Ti_2(Si_6O_(17))_2O_2(OH)_6F(H_2O)_2, from Mt. Kukisvumchorr, Khibiny alkaline massif, Kola peninsula, Russia, a 5.358(1), b 16.204(3), c 21.976(4) ?, β 94.91(1)°, V 1901.0(7) ?~3, space group A2/m, Z = 2, D_(calc). 3.116 g/cm3, has been refined to an R1 value of 5.60 for 2747 observed F_o >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 Fe~(3+)/(Fe~(2+)Fe~(3+)) ratio was measured by M?ssbauer spectroscopy, and FTIR and Raman spectra were collected. The empirical formula was calculated on the basis of (O + F + OH + H_2O) = 44.39 pfu: (Na_(1.39)K_(0.61))_(∑2)Na_1(Rb_(0.06)Cs_(0.02)□_(0.92))_(∑1)(Fe_9.11~(2+)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))_(∑2)O_34O_2(OH)_6 (F_(0.86)O_(0.14))_(∑1)(H2O)_(1.39); Z = 2; the content of H_2O 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 Fe2t-dominant M(1-3) octahedra, the M(1-3) sites are fully occupied and the ideal composition of the O sheet is Fe_10~(2+) apfu. The H sheet is composed of the nafertisite T_6O_(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_6O_(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_2O)_(1.39)□_(0.61) pfu, respectively. The chemical composition of the I block of the general formula A_2BCW is (Na_(1.39) K_(0.61))Na_1(□_(0.92)Rb_(0.06)Cs_(0.02))(H_2O)_(1.39) pfu, ideally Na_3(H_2O)_2 pfu. Nafertisite is closely related to the astrophyllite-group minerals.