• journal_title：The Canadian Mineralogist
• Contributor：Christine M. Clark ; Frank C. Hawthorne ; Luisa Ottolini
• Publisher：Mineralogical Association of Canada
• Date：2011-
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.49.1.57
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：49
• issue：1
• firstpage：57
• section：Articles

Fluor-dravite, NaMg₃Al₆Si₆O₁₈(BO₃)₃(OH)₃F, 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, ω= 1.645(2), ɛ= 1.621(2). Fluor-dravite is rhombohedral, space group R3m, a = 15.955(3), c = 7.153(2) Å, V = 1576.9(6) ų, Z = 3. The strongest ten X-ray-diffraction lines in the powder pattern [d in Å(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, B₂O_{3 calc} 10.86, Li₂O_calc 0.20, F 1.45, H₂O 3.19, sum 100.94 wt%. The formula unit is (Na_0.88Ca_0.04)_∑0.92 (Mg_1.87Fe_0.87Mn_0.09Zn_0.01Ti_0.03Li_0.13)_∑3.00 Al₆ (Si_5.87B_0.05Al_0.08) O₁₈ (BO₃)₃ (OH)₃ [(OH)_0.47F_0.75)]_∑1.22. Fluor-dravite, ideally NaMg₃Al₆Si₆O₁₈(BO₃)₃ (OH)₃F, is related to end-member dravite, ideally NaMg₃Al₆Si₆O₁₈(BO₃)₃(OH)₃(OH), by the substitution F → (OH).