Torrecillasite, Na(As,Sb)³⁺ ₄O₆Cl, a new mineral from the Torrecillas mine, Iquique Province, Chile: description and crystal structure

The new mineral torrecillasite (IMA2013-112), Na(As, Sb)³⁺ ₄O₆Cl, was found at the Torrecillas mine, Iquique Province, Chile, where it occurs as a secondary alteration phase in association with anhydrite, cinnabar, gypsum, halite, lavendulan, magnesiokoritnigite, marcasite, quartz, pyrite, scorodite, wendwilsonite and other potentially new As-bearing minerals. Torrecillasite occurs as thin colourless prisms up to 0.4 mm long in jack-straw aggregates, as very thin fibres in puff balls and as massive intergrowths of needles. Prisms are elongated on [100] with diamond-shaped cross-section and irregular terminations. Crystals are transparent, with adamantine lustre and white streak. The Mohs hardness is 2½, tenacity is brittle and fracture is irregular. Cleavage on (001) is likely. The calculated density is 4.056 g cm^–3. Optically, torrecillasite is biaxial (–) with α = 1.800(5), β = 1.96(1), γ = 2.03(calc.) (measured in white light). The measured 2V is 62.1(5)°, no dispersion or pleochroism were observed, the optical orientation is X = c, Y = b, Z = a. The mineral is very slowly soluble in H₂O, slowly soluble in dilute HCl and rapidly soluble in concentrated HCl. The empirical formula, determined from electron-microprobe analyses, is (Na_1.03Mg_0.02)_Σ1.05(As_3.39Sb_0.62)_Σ4.01O_6.07Cl_0.93. Torrecillasite is orthorhombic, Pmcn, a = 5.2580(9), b = 8.0620(13), c = 18.654(3) Å, V = 790.7(2) ų and Z = 4. The eight strongest X-ray powder diffraction lines are [d _obs Å (I)(hkl)]: 4.298(33)(111), 4.031(78)(014,020), 3.035(100)(024,122), 2.853(39)(115,123), 2.642(84)(124,200), 2.426(34)(125), 1.8963(32)(225) and 1.8026(29)(0·1·10,233). The structure, refined to R ₁ = 4.06% for 814 F _o >4σF reflections, contains a neutral, wavy As₂O₃ layer parallel to (001) consisting of As³⁺O₃ pyramids that share O atoms to form six-membered rings. Successive layers are flipped relative to one another and successive interlayer regions contain alternately either Na or Cl atoms. Torrecillasite is isostructural with synthetic orthorhombic NaAs₄O₆Br.