Naalasite, NaAl(AsO 3 OH) 2 ⋅H 2 O, the Al analogue of nafeasite from the Torrecillas mine, Iquique Province, Chile

The new mineral naalasite (IMA2023–027), NaAl(AsO 3 OH) 2 ⋅H 2 O, 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 Fe 3+ 0.39 As 2 O 9 H 4.07 . Naalasite is trigonal, space group R 32, with cell parameters: a = 8.494(4), c = 26.430(13) Å, V = 1651.5(4) Å 3 and Z = 9. The structure, refined to R 1 = 3.78% for 641 I > 2σ I reflections, is based on a loose 3D framework of alternating AsO 3 OH tetrahedra and AlO 6 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 C 2.