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Torrecillasite, Na(As,Sb)43+O6Cl, a new mineral from the Torrecillas mine, Iquique Province, Chile: description and crystal structure

Published online by Cambridge University Press:  05 July 2018

A. R. Kampf ,
B. P. Nash ,
M. Dini  and
A. A. Molina Donoso
A. R. Kampf*
Affiliation:
Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA 90007, USA
B. P. Nash
Affiliation:
Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, USA
M. Dini
Affiliation:
Pasaje San Agustin 4045, La Serena, Chile
A. A. Molina Donoso
Affiliation:
Los Algarrobos 2986, Iquique, Chile
*
E-mail: [email protected]
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Abstract

The new mineral torrecillasite (IMA2013-112), Na(As,Sb)43+O6Cl, 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 (Na1.03Mg0.02)∑1.05(As3.39Sb0.62)∑4.01O6.07Cl0.93. Torrecillasite is orthorhombic, Pmcn, a = 5.2580(9), b = 8.0620(13), c = 18.654(3) Å, V = 790.7(2) Å3 and Z = 4. The eight strongest X-ray powder diffraction lines are [dobs 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 R1 = 4.06% for 814 Fo >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.

Keywords

torrecillasitenew mineralarsenitecrystal structureTorrecillas mineChile
Type
Research Article
Information
Mineralogical Magazine , Volume 78 , Issue 3 , June 2014 , pp. 747 - 755
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2014

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