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Abstract:

Lalondeite, ideally (Na, Ca)(6)(Ca, Na)(3)Si(16)O(38)(F,OH)(2)center dot 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 <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 similar to 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(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(meas) = 48(1)degrees, 2V(calc) = 44(1)degrees; no dispersion was noted. The optical orientation is X approximate to 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))(Sigma 6.46)Ca(3)Si(16.16)O(38)(F(0.99)OH(0.94)Cl(0.07))(Sigma 2)center dot 3H(2)O or, ideally, (Na, Ca, square)(6)Ca(3)Si(16) O(38)(F,OH)(2)center dot 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)over bar>, a 9.589(2), b 9.613(2), c 12.115(2) angstrom, alpha 96.62(2), beta 92.95(2), gamma 119.81(2)degrees, V 954.8(1) angstrom(3), Z = 2. The strongest seven lines in the X-ray powder-diffraction pattern [d in angstrom(I)(hkl)] are: 11.938(90)(001), 4.142(30)(2 (2) over bar0), 4.106(30)(020,0 (2) over bar1), 3.972(40)(003,1 (2) over bar2), 2.981(35)(2 (1) over bar3,004), 2.967(50)((12) over bar2,3 (1) over bar1,(1) over bar 04,(2) over bar 31) and 2.888(100)((1) over bar 23,121,2 (3) over bar2,(3) over bar 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 SiO(4) tetrahedra cross-linked by SiO(4) tetrahedra, and the O layers contain edge-sharing M phi(6-7) octahedra and polyhedra (M: Na, Ca; phi: unspecified ligand). The O sheets are sandwiched between two symmetrically equivalent T layers (T(2), (T) over bar (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) over bar (2)O module, similar to that found in specific members of the reyerite-gyrolite group.

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