Wiluite, Ca-19(Al,Mg,Fe,Ti)(13)(B,Al square)(5)Si18O68(O,OH)(10), a new mineral species isostructural with vesuvianite, from the Sakha Republic, Russian Federation

Wiluite, Ca-19(Al,Mg,Fe,Ti)(13)(B,Al,square)(5)Si18O68(OH,F,O)(10), tetrago nal, a 15.752(1), c 11.717(1) Angstrom, V 2907.3(3) Angstrom(3), space grou p P4/nnc, Z = 2, is a new mineral species of the vesuvianite group from the Wilui River, Sakha Republic, Russian Federation. It occurs as holosymmetri c crystals with forms {100} and {101} dominant plus minor {110} and {001}, up to 3 x 3 x 5 cm, slightly elongate along [001]. No twinning was observed . Wiluite is dark green with a colorless streak, a vitreous luster, and it does not fluoresce under long- or short-wave ultraviolet light. It has a Mo hs hardness of 6, is brittle with an irregular fracture, and has poor cleav age on {100}. The measured density is 3.36(3) g/cm(3), and the calculated d ensity is 3.358 g/cm(3). In transmitted light, wiluite is colorless and non pleochroic; it is uniaxial positive with omega = 1.721(2) and epsilon = 1.7 25(2). The strongest ten reflections in the X-ray powder-diffraction patter n [d(in Angstrom)(I)(hkl)] are 2.776(100)(342), 2.617(61)(252), 2.491(61)(2 60), 2.592(43)(224), 1.66(26)(346), 1.640(23)(292), 2.121(20)(135), 3.52(18 )(240), 1.987(15)(561), 2.929(14)(004), and 2.912(14)(233). An analysis of wiluite gave SiO2 36.11, Al2O3 12.03, TiO2 0.80, MgO 6.48, FeO 1.21, Fe2O3 2.28, CaO 35.54, Ce2O3 0.18, B2O3 3.06, F 0.50, H2O 0.61, O = F -0.24, sum = 99.20 wt.%. The concentration of FeO was determined by wet chemistry, and that of H2O was determined with a LECO induction furnace; the concentratio n of the other elements was determined by electron-microprobe analysis. The corresponding unit formula, based on 19(Ca + Ce), is (Ca18.97Ce0.03)(Sigma 19) (Al6.53Ti0.30Fe86Mg4.81Fe50)(Sigma 13)(B2.63Al0.53-1.84)(Sigma 5)Si17. 99O68 [(OH)(2.03)O-7.97](Sigma 10). Wiluite is isostructural with vesuviani te, and contains essential B [B > 2.5 apfu (atoms per formula unit)]; vesuv ianite is uniaxial (-), wiluite is uniaxial (+). It occurs with grossular a nd serpentine minerals in st serpentinite. The name recalls the locality, a nd has been in use as a varietal name for "vesuvianite" from this locality for over 200 years.