Naalasite, NaAl(AsO₃OH)₂⋅H₂O, the Al analogue of nafeasite from the Torrecillas mine, Iquique Province, Chile

Abstract

The new mineral naalasite (IMA2023–027), NaAl(AsO₃OH)₂⋅H₂O, was found at the Torrecillas mine, Iquique Province, Chile, where it is a secondary alteration phase associated with anhydrite, juansilvaite, magnesiokoritnigite and a lavendulan-like phase. Naalasite occurs in tightly intergrown aggregates and druses of equant crystals. Crystals are light to medium pink and transparent, with vitreous lustre and white streak. The Mohs hardness is ~3½. The density is 3.19(2) g⋅cm⁻³. Optically, naalasite is uniaxial (+), with ω = 1.630(3) and ɛ = 1.660(3) (white light). The empirical formula (based on 9 O apfu) is Na_(0.92)Al_(0.61)Fe³⁺_(0.39)As₂O₉H_(4.07). Naalasite is trigonal, space group R32, with cell parameters: a = 8.494(4), c = 26.430(13) Å, V = 1651.5(4) ų and Z = 9. The structure, refined to R₁ = 3.78% for 641 I > 2σ_I reflections, is based on a loose 3D framework of alternating AsO₃OH tetrahedra and AlO₆ octahedra. The structure is topologically equivalent to that of nafeasite and can be regarded as its Al analogue, even though nafeasite is monoclinic with space group C2.

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