Canutite, NaMn_3[AsO_4][AsO_3(0H)]_2, a new protonated alluaudite-group mineral from the Torrecillas mine, Iquique Province, Chile

摘要

The new mineral canutite (IMA2013-070), NaMn_3[AsO_4][AsO_3(0H)]_2, was found at two different locations at the Torrecillas mine, Salar Grande, Iquique Province, Chile, where it occurs as a secondary alteration phase in association with anhydrite, halite, lavendulan, magnesiokoritnigite, pyrite, quartz and scorodite. Canutite is reddish brown in colour. It forms as prisms elongated on [201] and exhibiting the forms {010}, {100}, {102}, {201} and {102}, or as tablets flattened on {102} and exhibiting the forms {102} and {110}. Crystals are transparent with a vitreous lustre. The mineral has a pale tan streak, Mohs hardness of 214, brittle tenacity, splintery fracture and two perfect cleavages, on {010} and {101}. The calculated density is 4.112 g cm~(-3). Optically, canutite is biaxial (+) with a = 1.712(3), P = 1.725(3) and y = 1.756(3) (measured in white light). The measured 2V is 65.6(4)°, the dispersion is r < v (slight), the optical orientation is Z = b; X~^ a = 18° in obtuse P and pleochroism is imperceptible. The mineral is slowly soluble in cold, dilute HCl. The empirical formula (for tabular crystals from near the mine shaft), determined from electron-microprobe analyses, is (Na_(1.05)Mn_(2.64)Mg_(0.34)Cu_(0.14)Co_(0.03))∑_(4.20)As_3O_(12)H_(1.62). Canutite is monoclinic, C2lc, a = 12.3282(4), b = 12.6039(5), c = 6.8814(5) A, p = 113.480(8)°, V= 980.72(10) A3 and 2= 4. The eight strongest X-ray powder diffraction lines are [d_(obs) A(I)(hkl)]: 6.33(34)(020), 4.12(26)(221), 3.608(29)(310,131), 3.296(57)(112), 3.150(28)(002,131), 2.819(42)(400,041,330), 2.740(100)(240,402,112) and 1.5364(31)(multiple). The structure, refined to R_1 = 2.33% for 1089 F_o > 4σF reflections, shows canutite to be isostructural with protonated members of the alluaudite group.