LAURENTIANITE, [NbO(H2O)]3(Si2O7)2[Na(H2O)2]3, A NEW MINERAL FROM MONT SAINT-HILAIRE, QUEBEC: DESCRIPTION, CRYSTAL-STRUCTURE DETERMINATION AND PARAGENESIS

Autor: Monika M. M. Haring, Andrew M. McDonald, Mark A. Cooper, Glenn A. Poirier
Rok vydání: 2012
Předmět:
Zdroj: The Canadian Mineralogist. 50:1265-1280
ISSN: 1499-1276
0008-4476
Popis: Laurentianite, [NbO(H 2 O)] 3 (Si 2 O 7 ) 2 [Na(H 2 O) 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/cm 3 . 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: Na 2 O 8.88 (4.54–12.80), K 2 O 0.26 (0.14–0.44), CaO 0.22 (0.10–0.43), TiO 2 0.58 (0.31–0.83), Nb 2 O 5 43.64 (36.43–49.90), SiO 2 26.87 (22.81–29.07), and H 2 O (calc.) 17.93, total 98.38 wt.% on the basis of 26 anions, corresponding to [(Nb 0.99 Ti 0.01 ) ∑1.00 O(H 2 O)] 3 (Si 2.00 O 7 ) 2 [(Na 0.86 □ 0.10 K 0.02 Ca 0.01 ) ∑0.99 (H 2 O) 2 ] 3 or, ideally, [NbO(H 2 O)] 3 (Si 2 O 7 ) 2 [Na(H 2 O) 2 ] 3 . The presence of H 2 O in laurentianite is inferred from Raman spectroscopy and results from refinement of the crystal structure. The mineral crystallizes in space group P 3 (#143) with a 9.937(1), c 7.004(1) A, V 599.0(1) A 3 , and Z = 1. The strongest six lines on the X-ray powder-diffraction pattern [ d in A (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 > 4σ F o ) contains one Na , two Nb , and four Si sites. The two Nb sites are coordinated in distorted Nb O 5 (H 2 O) octahedra, with four equatorial bonds of typical Nb–O bond distances (~2 A) and two highly asymmetric ones (one long, ~2.5 A and one short, ~1.8 A). Each site is each only partially occupied (~50%) and because of the short distance between them (~0.7 A), they are not simultaneously occupied. A novel cation-anion coordination scheme involving the apical oxygens, Nb, and disordered H 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 2− and the other by H 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 2− /H 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 2 O groups associated with the Na cation. The crystal structure of laurentianite is based on five-membered pinwheels of composition [Nb 3 Si 2 O 17 (H 2 O) 3 ] −11 , consisting of three Nb O 5 (H 2 O) octahedra linked to two SiO 4 tetrahedra. Individual Nb–Si pinwheels are attached to form a layer composed of 18-membered rings of composition [Nb 6 Si 12 O 54 (H 2 O) 6 ] 30− perpendicular to [001]. The crystal structure is also layered along [001], with a silicate layer composed of (Si 2 O 7 ) dimers and a layer of isolated Nb O 5 (H 2 O) 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.
Databáze: OpenAIRE
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