Nafeasite, NaFe³⁺(AsO₃OH)₂⋅H₂O, a new framework arsenate from the Torrecillas mine, Iquique Province, Chile

Abstract

The new mineral nafeasite (IMA2021-103), NaFe³⁺(AsO₃OH)₂⋅H₂O, was found at the Torrecillas mine, Iquique Province, Chile, where it is a secondary alteration phase associated with anhydrite, gypsum, halite, lavendulan, magnesiokoritnigite and natrojarosite. Nafeasite occurs in tightly intergrown aggregates of equant crystals. Crystals are light to medium pink and transparent, with vitreous lustre and white streak. The Mohs hardness is ~2½. The density is 3.23(2) g⋅cm⁻³. Optically, nafeasite is biaxial (+), with α = 1.679(3), β = 1.682(3), γ = 1.730(5) (white light); 2V = 27(2)°; and slight r < v dispersion. The empirical formulae of the holotype and cotype (based on 9 O atoms per formula unit) are Na_(0.98)K_(0.02)Fe_(0.92)Al_(0.07)As_(2.00)O₉H_(4.01) and Na_(0.97)Fe_(0.68)Al_(0.33)As_(2.00)O₉H_(4.01), respectively. Nafeasite is monoclinic, space group C2, with cell parameters: a = 18.6876(16), b = 8.6769(7), c = 14.8100(10) Å, β = 105.238 (5)°, V = 2317.0(3) ų and Z = 12. The structure, refined to R₁ = 5.03% for 5979 I_o > 2σI reflections, is based on a loose 3D framework of alternating AsO₃OH tetrahedra and Fe³⁺O₆ octahedra.

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