Abstract

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 × 0.012 × 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/cm3. Laurentianite is nonpleochroic and uniaxial negative, with ω 1.612(2) and ɛ 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)∑1.00O(H2O)]3 (Si2.00O7)2[(Na0.860.10K0.02Ca0.01)∑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) Å, V 599.0(1) Å3, and Z = 1. The strongest six lines on the X-ray powder-diffraction pattern [d in Å (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 (Fo > 4σFo) 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 (~2 Å) and two highly asymmetric ones (one long, ~2.5 Å and one short, ~1.8 Å). Each site is each only partially occupied (~50%) and because of the short distance between them (~0.7 Å), 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.

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 ~z + ½. 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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