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LALONDEITE, A NEW HYDRATED Na–Ca FLUOROSILICATE SPECIES FROM MONT SAINT-HILAIRE, QUEBEC: DESCRIPTION AND CRYSTAL STRUCTURE

Andrew M. McDonald^1, and George Y. Chao^2,¶

¹ Mineral Exploration Centre and Department of Earth Sciences, Laurentian University, Sudbury, Ontario P3E 2C6, Canada
² Ottawa–Carleton Geoscience Centre, Department of Earth Sciences, Carleton University, Ottawa, Ontario K1S 5B6, Canada

E-mail address: amcdonald{at}laurentian.ca

^¶ Present address: 2031 Delmar Drive, Ottawa, Ontario K1H 5P6, Canada.

Lalondeite, ideally (Na,Ca)₆(Ca,Na)₃Si₁₆O₃₈(F,OH)₂·3H₂O, is a new mineral species from the Poudrette quarry, Mont Saint- Hilaire, Quebec. The mineral forms elongate aggregates of densely packed, randomly oriented crystals. Individual crystals are colorless, subhedral, roundish plates, typically measuring <0.5 mm in thickness and up to three mm across, dominated by the pinacoid {001}. Associated minerals include microcline, clinoamphibole and narsarssukite. The mineral has a vitreous luster, and is transparent with a white streak. It shows no fluorescence under long-wave ultraviolet radiation, a medium to strong violet-blue fluorescence under medium-wave radiation, and a weak violet to violet-blue fluorescence under short-wave radiation. It has a Mohs hardness of ~3, an inelastic, perfect {001} cleavage, and is brittle with a splintery fracture; its density is 2.50(1) (measured) and 2.51 g/cm³ (calculated). Lalondeite is non-pleochroic, biaxial negative, with 1.522(1), β 1.528(1), 1.529(1) (for = 589 nm), 2V_meas = 48(1)°, 2V_calc = 44(1)°; no dispersion was noted. The optical orientation is X c. The average result of eighteen analyses of three crystals made using combined energy- and wavelength-dispersive spectrometries gave: Na₂O 11.28, K₂O 0.70, CaO 15.00, SiO₂ 66.36, F 1.28, Cl 0.18 and H₂O (calc.) 4.27, O = F + Cl –0.58, total 98.49 wt.%. The empirical formula, based on 43 anions (O + F + Cl), is: (Na_5.33Ca_0.91K_0.22)_6.46Ca₃Si_16.16O₃₈(F_0.99OH_0.94Cl_0.07)₂·3H₂O or, ideally, (Na,Ca,)₆Ca₃Si₁₆ O₃₈(F,OH)₂·3H₂O. The mineral does not effervesce in 1:1 HCl at room temperature. The principal absorption bands in the infrared spectrum include 3443, 1631 and 1025 cm^–1 (shoulder at 1121 cm^–1) with four bands in the region of 787–388 cm^–1. The mineral is triclinic, space group P, a 9.589(2), b 9.613(2), c 12.115(2) Å, 96.62(2), β 92.95(2), 119.81(2)°, V 954.8(1) ų, Z = 2. The strongest seven lines in the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are: 11.938(90)(001), 4.142(30)(20), 4.106(30)(020,021), 3.972(40)(003,12), 2.981(35)(23,004), 2.967(50)( 2,31, 04, 31) and 2.888(100)( 23,121,22, 12). The crystal structure of lalondeite is strongly layered along [001], with sheets of tetrahedra (T) and octahedra (O). The T layers are composed of six-membered rings of SiO₄ tetrahedra cross-linked by SiO₄ tetrahedra, and the O layers contain edge-sharing M_6–7 octahedra and polyhedra (M: Na,Ca; : unspecified ligand). The O sheets are sandwiched between two symmetrically equivalent T layers (T₂,₂), producing a strongly bonded T–O–T unit. The interlayer region houses disordered Na ions and H₂O molecules. Stacking of the principal components perpendicular to [001] results in a OT₂₂O module, similar to that found in specific members of the reyerite–gyrolite group.

Keywords: lalondeite, new mineral species, layered fluorosilicate hydrate, reyerite–gyrolite group, crystal structure, Mont Saint-Hilaire, Quebec.