Fluor-dravite, NaMg3Al6Si6O18(BO3)3(OH)3F, a new mineral species of the tourmaline group from the Crabtree emerald mine, Mitchell County, North Carolina: Description and crystal structure

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Geography and Geology

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Canadian Mineralogist


Fluor-dravite, NaMg3Al6Si6O18(BO3)(3)(OH)(3)F, is a new mineral species of the tourmaline group from the Crabtree emerald mine, Mitchell County, North Carolina, U. S. A. It occurs as isolated anhedral grains up to a few mm across at or near the boundary of a small body of granitic pegmatite with the surrounding country-rock, and is associated with plagioclase, K-feldspar, quartz, beryl, muscovite, garnet, biotite and fluorite. Crystals are blackish brown with a pale brown streak. Fluor-dravite is brittle and has a Mohs hardness of 7; it is non-fluorescent, has no observable cleavage or parting, and has a calculated density of 3.120 g cm(-3). In plane-polarized light, it is pleochroic, O = pale yellow-brown, E = colorless. Fluor-dravite is uniaxial negative, omega = 1.645(2), epsilon = 1.621(2). Fluor-dravite is rhombohedral, space group R3m, a = 15.955(3), c = 7.153(2) angstrom, V = 1576.9(6) angstrom(3), Z = 3. The strongest ten X-ray-diffraction lines in the powder pattern [d in angstrom(I)(hkl)] are: 3.475(100)(102), 2.583(67)(051), 2.961(60) (212), 1.920(27)(432), 3.998(22)(220), 6.375(19)(011), 2.043(19)(512), 2.392(14)(003), 2.123(14)(303) and 1.660(14)(063). Analysis by a combination of electron microprobe, SIMS and crystal-structure refinement gives SiO2 36.02, Al2O3 31.69, TiO2 0.25, FeO 6.41, MnO 0.67, ZnO 0.05, MgO 7.71, CaO 0.25, Na2O 2.80, B2O3 calc 10.86, Li2Ocalc 0.20, F 1.45, H2O 3.19, sum 100.94 wt%. The formula unit is (Na0.88Ca0.04)Sigma(0.92) (Mg1.87Fe0.87Mn0.09Zn0.01Ti0.03Li0.13) Sigma(3.00) Al-6 (Si5.87B0.05Al0.08) O-18 (BO3)(3) (OH)(3) [(OH)(0.47)F-0.75)]Sigma(1.22). Fluor-dravite, ideally NaMg3Al6Si6O18(BO3)(3) (OH)(3)F, is related to end-member dravite, ideally NaMg3Al6Si6O18(BO3)(3)(OH)(3)(OH)->, by the substitution F (OH).

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