LECOQITE-(Y), Na3Y(CO3)3{middle dot}6H2O, A NEW MINERAL SPECIES FROM MONT SAINT-HILAIRE, QUEBEC, CANADA

Abstract

Lecoqite-(Y), ideally Na 3 Y(CO 3 ) 3 ·6H 2 O, a new mineral species, was discovered at the Poudrette quarry, Mont Saint-Hilaire, Quebec, Canada. It is associated with microcline, albite, natrolite, gonnardite, aegirine, siderite, elpidite, gaidonnayite, zircon, franconite, dawsonite, rhodochrosite, cryolite, rutile, and sphalerite. Lecoqite-(Y) forms radiating, spray-like aggregates in compact, tightly packed masses to 2.5 cm across, composed of flexible, extremely thin, capillary crystals up to 2.5 cm long and up to 0.01 mm thick. Individuals are colorless, and aggregates are white. The luster is strikingly silky. D(calc.) = 2.358 g/cm 3 . Lecoqite-(Y) is optically uniaxial positive, ω = 1.521(3), e = 1.497(3). The IR spectrum is unique. The chemical composition (electron microprobe, H 2 O by modified Penfield method, CO 2 by selective absorption, average results) is: Na 2 O 19.22, CaO 0.03, Y 2 O 3 17.95, Nd 2 O 3 0.54, Sm 2 O 3 0.41, Gd 2 O 3 0.75, Dy 2 O 3 3.31, Ho 2 O 3 1.12, Er 2 O 3 3.20, Yb 2 O 3 1.48, CO 2 27.0, H 2 O 23.4, total 98.41 wt.%. The empirical formula calculated for 15 O apfu is: Na 2.94 (Y 0.755 Dy 0.085 Er 0.08 Yb 0.035 Ho 0.03 Gd 0.02 Nd 0.015 Sm 0.01 ) ∑1.03 (CO 3 ) 2.91 (OH) 0.21 (H 2 O) 6.06 . Lecoqite-(Y) is hexagonal, P 6 3 , a 11.316(4), c 5.931(2) A, V 657.7(4) A 3 , Z = 2. The crystal structure was established from X-ray powder data by the Rietveld method, based on the model of the isostructural synthetic Ln-free carbonate Na 3 Y(CO 3 ) 3 ·6H 2 O. Final agreement factors are: R p = 0.0468, R wp = 0.0657, R Bragg = 0.0343, R F = 0.0356. No mineral is closely related to lecoqite-(Y) in terms of structure. In the structure of lecoqite-(Y), REE atoms (REE = Y + Ln) are surrounded by six O atoms of CO 3 groups and three H 2 O molecules to form a tricapped triangular prism. The Na atoms are centered in distorted octahedra [NaO 4 (H 2 O) 2 ], which link to form infinite corrugated chains along c . The strongest five lines of the X-ray powder pattern [ d in A (I)( hkl )] are: 9.82(57)(100), 5.081(100)(101), 3.779(39)(201), 2.627(39)(112), 2.471(37)(131). Lecoqite-(Y) is named in memory of P.E. Lecoq de Boisbaudran (1838–1912), outstanding French chemist and specialist in the spectroscopic analysis of minerals and synthetic compounds, who made a great contribution to the chemistry of the rare-earth elements. The Levinson suffix modifier -(Y) is in line with the dominance of yttrium over other rare-earth elements in the mineral. The cotype specimens are deposited in the Fersman Mineralogical Museum of Russian Academy of Sciences, Moscow, and the Canadian Museum of Nature, Ottawa.