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

Abstract

Lalondeite, ideally (Na,Ca) 6 (Ca,Na) 3 Si 16 O 38 (F,OH) 2 ·3H 2 O, 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 3 (calculated). Lalondeite is non-pleochroic, biaxial negative, with α 1.522(1), β 1.528(1), γ 1.529(1) (for λ = 589 nm), 2 V meas = 48(1)°, 2 V 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: Na 2 O 11.28, K 2 O 0.70, CaO 15.00, SiO 2 66.36, F 1.28, Cl 0.18 and H 2 O (calc.) 4.27, O = F + Cl −0.58, total 98.49 wt.%. The empirical formula, based on 43 anions (O + F + Cl), is: (Na 5.33 Ca 0.91 K 0.22 ) ∑6.46 Ca 3 Si 16.16 O 38 (F 0.99 OH 0.94 Cl 0.07 ) ∑2 ·3H 2 O or, ideally, (Na,Ca,□) 6 Ca 3 Si 16 O 38 (F,OH) 2 ·3H 2 O. 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 1, a 9.589(2), b 9.613(2), c 12.115(2) A, α 96.62(2), β 92.95(2), γ 119.81(2)° , V 954.8(1) A 3 , Z = 2. The strongest seven lines in the X-ray powder-diffraction pattern [ d in A( I )( hkl )] are: 11.938(90)(001), 4.142(30)(220), 4.106(30)(020,021), 3.972(40)(003,122), 2.981(35)(213,004), 2.967(50)( 122,311, 104, 231) and 2.888(100)( 123,121,232, 312). 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 SiO 4 tetrahedra cross-linked by SiO 4 tetrahedra, and the O layers contain edge-sharing M ϕ 6–7 octahedra and polyhedra ( M : Na,Ca; ϕ: unspecified ligand). The O sheets are sandwiched between two symmetrically equivalent T layers ( T 2 , T 2 ), producing a strongly bonded T – O – T unit. The interlayer region houses disordered Na ions and H 2 O molecules. Stacking of the principal components perpendicular to [001] results in a OT 2 T 2 O module, similar to that found in specific members of the reyerite–gyrolite group.