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Laurentianite, [NbO(H (sub 2) O)] (sub 3) (Si (sub 2) O (sub 7) ) (sub 2) [Na(H (sub 2) O) (sub 2) ] (sub 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 (super 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: Na (sub 2) O 8.88 (4.54-12.80), K (sub 2) O 0.26 (0.14-0.44), CaO 0.22 (0.10-0.43), TiO (sub 2) 0.58 (0.31-0.83), Nb (sub 2) O (sub 5) 43.64 (36.43-49.90), SiO (sub 2) 26.87 (22.81-29.07), and H (sub 2) O (calc.) 17.93, total 98.38 wt.% on the basis of 26 anions, corresponding to [(Nb (sub 0.99) Ti (sub 0.01) ) (sub Sigma 1.00) O(H (sub 2) O)] (sub 3) (Si (sub 2.00) O (sub 7) ) (sub 2) [(Na (sub 0.86) [] (sub 0.10) K (sub 0.02) Ca (sub 0.01) ) (sub Sigma 0.99) (H (sub 2) O) (sub 2) ] (sub 3) or, ideally, [NbO(H (sub 2) O)] (sub 3) (Si (sub 2) O (sub 7) ) (sub 2) [Na(H (sub 2) O) (sub 2) ] (sub 3) . The presence of H (sub 2) O 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) Aa, V 599.0(1) Aa (super 3) , and Z = 1. The strongest six lines on the X-ray powder-diffraction pattern [d in Aa (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 (sub o) > 4sigma F (sub o) ) contains one Na, two Nb, and four Si sites. The two Nb sites are coordinated in distorted NbO (sub 5) (H (sub 2) O) octahedra, with four equatorial bonds of typical Nb-O bond distances ( approximately 2 Aa) and two highly asymmetric ones (one long, approximately 2.5 Aa and one short, approximately 1.8 Aa). Each site is each only partially occupied ( approximately 50%) and because of the short distance between them ( approximately 0.7 Aa), they are not simultaneously occupied. A novel cation-anion coordination scheme involving the apical oxygens, Nb, and disordered H (sub 2) O 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 O (super 2-) and the other by H (sub 2) O. 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 (O (super 2-) /H (sub 2) O) 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 H (sub 2) O groups associated with the Na cation. The crystal structure of laurentianite is based on five-membered pinwheels of composition [Nb (sub 3) Si (sub 2) O (sub 17) (H (sub 2) O) (sub 3) ] (super -11) , consisting of three NbO (sub 5) (H (sub 2) O) octahedra linked to two SiO (sub 4) tetrahedra. Individual Nb-Si pinwheels are attached to form a layer composed of 18-membered rings of composition [Nb (sub 6) Si (sub 12) O (sub 54) (H (sub 2) O) (sub 6) ] (super 30-) perpendicular to [001]. The crystal structure is also layered along [001], with a silicate layer composed of (Si (sub 2) O (sub 7) ) dimers and a layer of isolated NbO (sub 5) (H (sub 2) O) octahedra. Sodium atoms are positioned within the silicate layer, occupy ng sites that almost directly overly the Nb sites but are displaced approximately 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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GeoRef, Copyright 2018, American Geological Institute.<br/>2013-010793<br/>laurentianite<br/>Poudrette Quarry<br/>vuonnemite