PARAVINOGRADOVITE, (Na, )2 [(Ti4+,Fe3+)4 {Si2 O6}2 {Si3 Al O10} (OH)4] H2O, A NEW MINERAL SPECIES FROM THE KHIBINA ALKALINE MASSIF, KOLA PENINSULA, RUSSIA: DESCRIPTION AND CRYSTAL STRUCTURE

Paravinogradovite is a new mineral species from Mount Kukisvumchorr, Khibina alkaline massif, Kola Peninsula, Russia. Paravinogradovite forms prismatic crystals elongate along [100] and up to 0.5–1.0 cm long; crystals in some cases occur in fan-shaped aggregates sporadically scattered throughout a matrix of feldspar, nepheline and natrolite in a strongly mineralized nepheline–feldspar pegmatite. Associated minerals are nepheline, K-feldspar, albite, analcime, natrolite, aegirine, biotite, chlorite, zircon, ilmenite, pyrochlore, ancylite-(Ce), nordstrandite, carbonate-fluorapatite, fluorite, galena and cerussite. Crystals of paravinogradovite are colorless to white, with a white streak and a luster that varies from vitreous to pearly. The mineral is translucent to transparent, and shows weak yellow-green fluorescence under 240–400 nm ultraviolet radiation. Paravinogradovite has a perfect cleavage on {001} and an indistinct cleavage on {010}. It is brittle, has a splintery fracture and a Mohs hardness of 5. Its observed and calculated densities are 2.77(2) and 2.76 g/cm 3 , respectively. It is biaxial negative with α 1.707(2), β 1.741(2), γ 1.755(2), 2 V (obs.) = 64(1)°, 2 V (calc.) = 64°, nonpleochroic with dispersion r > v , with Z ≈ b , X ∧ a = 30°. Paravinogradovite is triclinic, space group P 1, a 5.2533(1), b 8.7411(3), c 12.9480(5) A, α 70.466(1), β 78.472(1), γ 89.932(1)°, V 547.65(5) A 3 , Z = 1. The strongest seven lines in the X-ray powder-diffraction pattern [ d in A( I )(( hkl )] are: 3.182(100)(013,014), 5.88(65)(011,012), 11.9(58)(001), 4.35(38)(021,102), 5.98(35)(002), 3.085(29)(123) and 2.735(21)(122). Chemical analysis by electron microprobe gave SiO 2 43.54, Al 2 O 3 6.12, Fe 2 O 3 4.11, Nb 2 O 5 0.50, TiO 2 29.59, BeO 0.76, MgO 0.13, Na 2 O 7.77, K 2 O 0.87, H 2 O 6.23, sum 99.62 wt.%, where the amount of Be and the amount of H 2 O were determined by crystal-structure analysis, and the valence state of Fe was determined by Mossbauer spectroscopy. The resulting empirical formula on the basis of 26 anions (including OH = 4 apfu and excluding H 2 O) is (Na 2.293 K 0.169 ) (Ti 3.386 4+ Fe 0.471 3+ Mg 0.029 Nb 0.034 ) (Si 6.626 Al 1.098 Be 0.276 ) O 22 (OH) 4 (H 2 O) 1.16 . There are prominent endothermic effects at 280 and 460°C; the principal losses in weight are within the temperature ranges 150–400 (3.8%) and 400–600°C (2.8%), and the total loss in weight at 980°C is 7.1%. The principal absorptions in the infrared are as follows: 3520, 3330, 3240, 1633, 1105, 989, 940, 725, 691, 638, 599, 568, 523, 459 and 418 cm −1 , indicative of both OH and H 2 O in the structure. The name recognizes the close structural and chemical relations between paravinogradovite and vinogradovite, ideally Na 5 Ti 4 4+ (Si 7 Al) O 26 (H 2 O) 3 . The crystal structure of paravinogradovite was solved by direct methods and refined to an R 1 index of 4.5% based on 4373 observed [ F o > 4σ F ] unique reflections measured with Mo K α X-radiation and a Bruker P 4 diffractometer with a CCD detector. Four (SiO 4 ) tetrahedra form pyroxene-like [Si 2 O 6 ] chains, and three (SiO 4 ) tetrahedra and one (AlO 4 ) tetrahedron form vinogradovite-like [Si 3 AlO 10 ] chains parallel to [100]. ( M O 6 ) octahedra ( M ≈ Ti 4+ ) share common edges to form two distinct zig-zag brookite-like chains along [100]. One chain is decorated by ( X O 6 ) octahedral ( X ≈ Na) and linked into a sheet parallel to (100) by [Si 3 AlO 10 ] chains. The other distinct brookite-like chain is not decorated by ( X O 6 ) octahedra, but is linked into a sheet parallel to (100) by [Si 3 AlO 10 ] chains. Chains of tetrahedra and chains of octahedra link to form a framework with channels along [100]. These channels contain disordered (H 2 O) groups, the A (5) site partly occupied (14%) by K, and the A (1)– A (4) sites partly occupied (15–19%) by Na, giving a channel content of [Na 0.72 K 0.14 (H 2 O) 1.16 ]. The triclinic cell of paravinogradovite is related to the C -centered monoclinic cell of vinogradovite, ideally Na 5 Ti 4 4+ (Si 7 Al) O 26 (H 2 O) 3 [monoclinic, a 24.490(10), b 8.657(4), c 5.203(2) A, β 100.2(0)° , V 1085.8 A 3 , space group C 2/ c , Z = 2] by the matrix transformation (0 0 1, 0 1 0, −½ 1 0).

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