Paravinogradovite, (Na,square)(2) [(Ti4+,Fe3+)(4) (Si-2 O-6}(2) {Si-3 AlO10) (OH)(4)] H2O, a new mineral species from the Khibina Alkaline massif, Kola Peninsula, Russia: Description and crystal structure

Abstract

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 alpha 1.707(2), beta 1.741(2), gamma 1.755(2) 2V(obs.) = 64(1)degrees, 2V(calc.) = 64degrees, nonpleochroic with dispersion r > v, with Z approximate to b, X Lambda a = 30degrees. Paravinogradovite is triclinic, space group P1, a 5.2533(1), b 8.7411(3), c 12.9480(5) Angstrom, alpha 70.466( 1), beta 78.472(1), gamma 89.932(1)degrees, V 547.65(5) Angstrom(3), Z = 1. The strongest seven lines in the X-ray powder-diffraction pattern [d in Angstrom(I)((hkl)] are: 3.182(100)(0 (1) over bar3,014), 5.88(65)(0 (1) over bar1,012) 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 SiO2 43.54, Al2O3 6.12, Fe2O3 4.11, Nb2O5 0.50, TiO2 29.59, BeO 0.76, MgO 0.13, Na2O 7.77, K2O 0.87, H2O 6.23, sum 99.62 wt.%, where the amount of Be and the amount of H2O 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 H2O) is (Na-2.293 K-0.169) (Ti3.3864+Fe0.4713+Mg0.029Nb0.034) (Si6.626Al1.098Be0.276)O-22 (OH)(4) (H2O)(1.16). There are prominent endothermic effects at 280 and 460degreesC; the principal losses in weight are within the temperature ranges 150-400 (3.8%) and 400-600degreesC (2.8%), and the total loss in weight at 980degreesC 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 H2O in the structure. The name recognizes the close structural and chemical relations between paravinogradovite and vinogradovite, ideally Na5Ti44+(Si7Al)O-26(H2O)(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 > 4sigmaF] unique reflections measured with MoKalpha X-radiation and a Bruker P4 diffractometer with a CCD detector. Four (SiO4) tetrahedra form pyroxene-like [Si2O6] chains, and three (SiO4) tetrahedra and one (AlO4) tetrahedron form vinogradovite-like [Si3AlO10] chains parallel to [100]. (MO6) octahedra (M approximate to Ti4+) share common edges to form two distinct zig-zag brookite-like chains along [100]. One chain is decorated by (XO6) octahedra (X approximate to Na) and linked into a sheet parallel to (100) by [Si3AlO10] chains. The other distinct brookite-like chain is not decorated by (XO6) octahedra, but is linked into a sheet parallel to (100) by [Si3AlO10] chains. Chains of tetrahedra and chains of octahedra link to form a framework with channels along [100]. These channels contain disordered (H2O) 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 [Na0.72K0.14(H2O)(1.16)]. The triclinic cell of paravinogradovite is related to the C-centered monoclinic cell of vinogradovite, ideally Na-5 Ti-4(4+) (Si7Al) O-26 (H2O)(3) [monoclinic, a 24.490(10), b 8.657(4), c 5.203(2) Angstrom, beta 100.2(0)degrees, V 1085.8 Angstrom(3), space group C2/c, Z = 2] by the matrix transformation (001, 010, -(1)/(2) 10).