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Abstract:

Lalondeite, ideally (Na,Ca) (sub 6) (Ca,Na) (sub 3) Si (sub 16) O (sub 38) (F,OH) (sub 2) .3H (sub 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 <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 approximately 3, an inelastic, perfect {001} cleavage, and is brittle with a splintery fracture; its density is 2.50(1) (measured) and 2.51 g/cm (super 3) (calculated). Lalondeite is non-pleochroic, biaxial negative, with alpha 1.522(1), beta 1.528(1), gamma 1.529(1) (for lambda =589 nm), 2V (sub meas) =48(1) degrees , 2V (sub calc) =44(1) degrees ; no dispersion was noted. The optical orientation is X nearly equal c. The average result of eighteen analyses of three crystals made using combined energy- and wavelength-dispersive spectrometries gave: Na (sub 2) O 11.28, K (sub 2) O 0.70, CaO 15.00, SiO (sub 2) 66.36, F 1.28, Cl 0.18 and H (sub 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 (sub 5.33) Ca (sub 0.91) K (sub 0.22) ) (sub Sigma 6.46) Ca (sub 3) Si (sub 16.16) O (sub 38) (F (sub 0.99) OH (sub 0.94) Cl (sub 0.07) ) (sub Sigma 2) .3H (sub 2) O or, ideally, (Na,Ca,[]) (sub 6) Ca (sub 3) Si (sub 16) O (sub 38) (F,OH) (sub 2) .3H (sub 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 (super -1) (shoulder at 1121 cm (super -1) ) with four bands in the region of 787-388 cm (super -1) . The mineral is triclinic, space group P1, a 9.589(2), b 9.613(2), c 12.115(2) Aa, alpha 96.62(2), beta 92.95(2), gamma 119.81(2) degrees , V 954.8(1) Aa (super 3) , Z=2. The strongest seven lines in the X-ray powder-diffraction pattern [d in Aa(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 (sub 4) tetrahedra cross-linked by SiO (sub 4) tetrahedra, and the O layers contain edge-sharing Mphi (sub 6-7) octahedra and polyhedra (M: Na,Ca; phi : unspecified ligand). The O sheets are sandwiched between two symmetrically equivalent T layers (T (sub 2) ,T (sub 2) ), producing a strongly bonded T-O-T unit. The interlayer region houses disordered Na ions and H (sub 2) O molecules. Stacking of the principal components perpendicular to [001] results in a OT (sub 2) T (sub 2) O module, similar to that found in specific members of the reyerite-gyrolite group.

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GeoRef, Copyright 2018, American Geological Institute.<br/>2010-023196<br/>fluorosilicate<br/>lalondeite<br/>Mont Saint Hilaire<br/>Poudrette Quarry<br/>reyerite