Magnesiocanutite, NaMnMg₂[AsO₄]2[AsO₂(OH)₂], a new protonated alluaudite-group mineral from the Torrecillas mine, Iquique Province, Chile

Abstract

The new mineral magnesiocanutite (IMA2016-057), NaMnMg2[AsO4]2[AsO2(OH)2], was found at the Torrecillas mine, Iquique Province, Chile, where it occurs as a secondary phase in association with anhydrite, canutite, halite, lavendulan and magnesiokoritnigite. Magnesiocanutite occurs as pale brownish-pink to rose-pink, lozenge-shaped tablets that are often grouped in tightly intergrown aggregates. The crystal forms are {110} and {102}. Crystals are transparent, with vitreous lustre and white to very pale pink streak. The Mohs hardness is 2½, tenacity is brittle, and the fracture is splintery. Crystals exhibit two perfect cleavages: {010} and {101}. The calculated density is 3.957 g/cm3. Optically, magnesiocanutite is biaxial (+), with α = 1.689(2), β = 1.700(2), γ = 1.730(2) (measured in white light); 2Vmeas. = 64.3(4)°; slight dispersion, r <v; orientation Z = b; X ∧ a = 15° in obtuse angle β. The mineral is slowly soluble in dilute HCl at room temperature. Electron-microprobe analyses, provided Na2O 5.44, CaO 0.26, MgO 8.84, MnO 18.45, CoO 1.47, CuO 2.13, As2O5 59.51, H2O(calc) 2.86, total 98.96 wt.%. Magnesiocanutite is monoclinic, C2/c, a = 12.2514(8), b = 12.4980(9), c = 6.8345(5) Å, β = 113.167(8)°, V = 962.10(13) Å3 and Z = 4. The eight strongest powder X-ray diffraction lines are [dobs Å(I )(hkl)]: 6.25(42)(020), 3.566(43)(310,1̄31), 3.262(96)(1̄12), 3.120(59)(002,131,040,221), 2.787(93)(400,022,041,330), 2.718(100) (4̄21,240,112,402), 2.641(42)(1̄32) and 1.5026(43)(multiple). Magnesiocanutite has a protonated alluaudite-type structure (R1 = 2.59% for 789 Fo > 4σF reflections) and is the Mg analogue of canutite. Using the results of both the microprobe analyses and structure refinement, the structurally based empirical formula is Na(Mn0.78Mg0.22)Σ1.00(Mg1.04Mn0.70Cu0.15Co0.11)Σ2.00[AsO4]2[AsO2(OH)2].

Camanchacaite, chinchorroite, espadaite, magnesiofluckite, picaite and ríosecoite: six new hydrogen-arsenate minerals from the Torrecillas mine, Iquique Province, Chile

“…Camanchacaite is a member of the alluaudite group and is isostructural with compounds with the protonated alluaudite-type structure, including the minerals o'danielite, NaZn 3 [AsO 4 ][AsO 3 (OH)] 2 (Keller and Hess, 1988), groatite, NaCaMn 2 [PO 4 ][PO 3 (OH)] 2 (Cooper et al ., 2009), canutite, NaMn 3 [AsO 4 ][AsO 3 (OH)] 2 (Kampf et al ., 2014 b ), and magnesiocanutite, NaMnMg 2 [AsO 4 ] 2 [AsO 2 (OH) 2 ] (Kampf et al ., 2017 c ). The general formula of these phases is AM 1 M 2 2 [ T 1O 4 ][ T 2O 3 (OH)] 2 { T = P or As}, where the M 1 and M 2 octahedra link by edge-sharing to form staggered chains, T 1 and T 2 tetrahedra cross-link the chains, and A cations occupy large channel sites.…”