Laurentianite, [NbO(H2O)](3)(Si2O7)(2)[Na(H2O)(2)](3), is a new mineral discovered in siderite-dominant pods in an altered syenite at the Poudrette quarry, Mont Saint-Hilaire, Quebec. Crystals are colorless, acicular, euhedral, and elongate along [001] with average dimensions of 0.012 x 0.012 x 0.25 mm. The mineral generally occurs in loose, randomly oriented groupings ('nests') of crystals. Associated minerals include quartz, pyrite, franconite, rutile, lepidochrocite, and an unidentified Fe-bearing mineral. Laurentianite is transparent to translucent with a vitreous luster and is non-fluorescent under long-, medium-, and short-wave radiation. The Mohs hardness could not be measured owing to the small size of the crystals. No partings or cleavages were observed, although crystals do exhibit a splintery fracture. The calculated density is 2.464 g/cm(3). Laurentianite is nonpleochroic and uniaxial negative, with omega 1.612(2) and epsilon 1.604(2). The average of 12 analyses from several crystals is: Na2O 8.88 (4.54-12.80), K2O 0.26 (0.14-0.44), CaO 0.22 (0.10-0.43), TiO2 0.58 (0.31-0.83), Nb2O5 43.64 (36.43-49.90), SiO2 26.87 (22.81-29.07), and H2O (calc.) 17.93, total 98.38 wt.% on the basis of 26 anions, corresponding to [(Nb0.99Ti0.01)(Sigma 1.00)O(H2O)](3) (Si2.00O7)(2)[(Na-0.86 square 0.10K0.02Ca0.01)(Sigma 0.99)(H2O)(2)](3) or, ideally, [NbO(H2O)](3)(Si2O7)(2)[Na(H2O)(2)](3). The presence of H2O in laurentianite is inferred from Raman spectroscopy and results from refinement of the crystal structure. The mineral crystallizes in space group P3 (#143) with a 9.937(1), c 7.004(1) angstrom, V 599.0(1) angstrom(3), and Z = 1. The strongest six lines on the X-ray powder-diffraction pattern [d in angstrom (I) (hkl)] are: 8.608 (100) (010), 7.005 (19) (001), 4.312 (25) (020), 3.675 (25) (201, 021), 3.260 (31) (120, 210), and 2.870 (20) (030). The crystal structure of laurentianite, refined to R = 2.78% for 2347 reflections (F-o > 4 sigma F-o) contains one Na, two Nb, and four Si sites. The two Nb sites are coordinated in distorted NbO5(H2O) octahedra, with four equatorial bonds of typical Nb-O bond distances (similar to 2 angstrom) and two highly asymmetric ones (one long, similar to 2.5 angstrom and one short, similar to 1.8 angstrom). Each site is each only partially occupied (similar to 50%) and because of the short distance between them (similar to 0.7 angstrom), they are not simultaneously occupied. A novel cation-anion coordination scheme involving the apical oxygens, Nb, and disordered H2O groups is developed: when one of the Nb sites is occupied, the other is vacant, resulting in one of the apical O sites being occupied by O2- and the other by H2O. The opposite situation occurs when the occupancy and vacancy of the Nb sites are reversed, leading to both apical O sites having an equal, mixed (O2-/ H2O) composition. A minor charge understaturation at both apical O sites is remedied by each of these O sites receiving a single H-bond from one of the H2O groups associated with the Na cation.<br/>The crystal structure of laurentianite is based on five-membered pinwheels of composition [Nb3Si2O17(H2O)(3)](-11), consisting of three NbO5(H2O) octahedra linked to two SiO4 tetrahedra. Individual Nb-Si pinwheels are attached to form a layer composed of 18-membered rings of composition [Nb6Si12O54(H2O)(6)](30-) perpendicular to [001]. The crystal structure is also layered along [001], with a silicate layer composed of (Si2O7) dimers and a layer of isolated NbO5(H2O) octahedra. Sodium atoms are positioned within the silicate layer, occupying sites that almost directly overly the Nb sites but are displaced similar to z + 1/2. Laurentianite is a late-stage mineral intergrown with lepidocrocite, both of which overgrow franconite and quartz. The mineral is believed to have precipitated from a late-stage aqueous fluid enriched in Na, Si, and Nb, possibly arising through the breakdown of franconite, sodalite, and quartz.
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