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MARTINITE, A NEW HYDRATED SODIUM CALCIUM FLUORBOROSILICATE SPECIES FROM MONT SAINT-HILAIRE, QUEBEC: DESCRIPTION, STRUCTURE DETERMINATION AND GENETIC IMPLICATIONS

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

Martinite, ideally (Na,,Ca)₁₂Ca₄(Si,S,B)₁₄B₂O₃₈(OH,Cl)₂F₂·4H₂O, is a new mineral species from the Poudrette quarry, Mont Saint-Hilaire, Quebec. The mineral arose through the interaction of highly fractionated, hyperagpaitic fluids with sodalite syenite xenoliths. Martinite develops both as single crystals and as rosettes of triangular to roughly hexagonal plates varying from 50 µm to a maximum of about 1 mm across. Individual crystals are typically <10 µm in thickness and dominated by {001}, and colorless to mauve to purple in color. Associated minerals include aegirine, albite, erdite, eudialyte-group minerals, galena, langite, lueshite, lovozerite-group minerals, molybdenite, posnjakite, rasvumite, sérandite, sazhinite-(Ce), sphalerite, terskite, ussingite, villiaumite, wurtzite and a number of unknown minerals, including several Cu_2–xS minerals (UK55 group), UK53a, UK73, UK82, UK91 and a löllingite-like mineral. The mineral has a vitreous luster, is transparent, has a white streak and shows no fluorescence in either short- or long-wave ultraviolet radiation. It has a Mohs hardness of 4; the crystals are inelastic, and have a perfect cleavage on {001}. It is brittle with an uneven fracture. The calculated density is 2.51 g/cm³. Martinite is non-pleochroic, optically biaxial (–), with 1.529(1), β 1.549(1), 1.551(1) (for = 590 nm), 2V_meas = 38(1)°, 2V_calc = 35(1)°; no dispersion was noted. The optical orientation is X c. On average, 19 electron-microprobe analyses gave: Na₂O 17.70, MgO 0.03, CaO 16.71, MnO 0.07, B₂O₃ (calc.) 5.02, SiO₂ 48.85, TiO₂ 0.06, SO₃ 2.30, F 2.18, Cl 1.09 and H₂O (calc.) 4.44, O=F+Cl –1.17, total 97.28 wt.%. The empirical formula (based on 46 anions) is: (Na_9.191.99Ca_0.82)₁₂(Ca_3.97Mn_0.02Mg_0.01)₄ (Si_13.08S_0.46B_0.45Ti_0.01)_14.00B₂O₃₈ (OH_1.50Cl_0.50)_2.00 (F_1.84OH_0.16)₂·4H₂O. The principal absorption bands in the infrared spectrum include 3437, 1634 and 1011 cm^–1 (shoulders at 1137, 1081, 898 and 862 cm^–1) and five bands in the region 786–498 cm^–1. The mineral is triclinic, space group P, a 9.5437(7), b 9.5349(6), c 14.0268(10) Å, 108.943(1), β 74.154(1), 119.780(1)°, V 1038.1(1) ų, Z = 2. The strongest seven lines in the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are: 13.18(100)(001), 6.58(43)(002), 2.968(37)(10), 3.29(34)(004,20), 2.908(27)(33), 3.02(17)(1), 2.800(17)(12). The structure of martinite has been refined to R = 6.30, wR² = 13.82%. The mineral is strongly layered, with sheets of tetrahedra (T), octahedra (O) and interlayer cations (X). The T layers are composed of six-membered rings of SiO₄ tetrahedra linked by [(Si,B)]O₄ and (BO₄) tetrahedra. The sheets of octahedra are composed of edge-sharing M₆ octahedra (M: Na,Ca; : unspecified ligand) arranged in a closest-packed arrangement. The O sheets are effectively sandwiched between two symmetrically equivalent T layers (T₂, &Tmacr;₂), producing a strongly bonded T–O–T unit. The interlayer component (X) houses poorly ordered Na polyhedra and H₂O molecules. Stacking of these principal components perpendicular to [001] results in a OT₂X&Tmacr;₂O module. Martinite is a member of the reyerite–gyrolite group. It represents the first B-bearing mineral of the group and the first mineral known to possess the OT₂X&Tmacr;₂O module. The name honors Robert François Martin (b. 1941), Professor of Geology at McGill University and long-time editor of The Canadian Mineralogist.

Keywords: martinite, new mineral species, layered borosilicate, reyerite–gyrolite group, crystal structure, Mont Saint-Hilaire, Quebec, Canada.