Calamaite, a new natural titanium sulfate from the Alcaparrosa mine, Calama, Antofagasta region, Chile

The new mineral calamaite, Na₂TiO(SO₄)₂ · 2H₂O, was found in the oxidation zone of a pyrite orebody at the abandoned Alcaparrosa sulfate mine, Calama commune, El Loa province, Antofagasta region, Chile. It is associated with römerite, coquimbite, metavoltine, tamarugite, halotrichite, szomolnokite, rhomboclase, and ferricopiapite. Calamaite forms acicular to hair-like crystals up to 0.01 × 2 mm combined in bunches or radial spherulitic clusters up to 4 mm across; rarely prismatic crystals up to 1 × 1 × 3 mm occur. Cross-like interpenetration twins are common. Calamaite is transparent, colourless in separate crystals and white in aggregates, with vitreous lustre. The mineral is brittle, with Mohs' hardness ca. 3. Good cleavage, presumably on (0 0 1), was observed. D_calc is 2.45 g · cm⁻³. Calamaite is optically biaxial (+), α = 1.557(2), β = 1.562(2), γ = 1.671(3), 2V_meas = 30(10)°. The IR spectrum is reported. The chemical composition (wt%, electron microprobe data; H₂O content is calculated for 2H₂O pfu) is: Na₂O 18.21, K₂O 0.06, Fe₂O₃ 1.58, TiO₂ 21.80, SO₃ 48.25, H₂O_calc 10.74, total 100.73. The empirical formula calculated on the basis of 11 O apfu is: $Na1.97(Ti0.92Fe0.073+)∑0.99∑2.02O9·2H2O$ Calamaite is orthorhombic, Ibam, a = 16.0989(11), b = 16.2399(9), с = 7.0135(4) Å, V = 1833.6(2) ų, and Z = 8. The strongest reflections of the powder XRD pattern [d,Å(I)(hkl)] are: 8.10(1 0 0)(0 2 0, 2 0 0), 5.04(55)(1 2 1, 2 1 1), 3.787(26)(2 3 1), 3.619(18)(2 4 0, 4 2 0), 3.417(27)(1 4 1, 4 1 1), 3.185(15)(1 5 0), 2.943(20)(3 4 1, 4 3 1), and 2.895(20)(1 3 2, 3 1 2). Calamaite represents a novel structure type. Its crystal structure was solved from single-crystal XRD data (R = 0.0358). The TiO₆ octahedra are interconnected via O vertices to form infinite (Ti–O)_∝ chains. The remaining vertices of each TiO₆ octahedron are shared with SO₄ tetrahedra thus forming one-dimensional [TiO(SO₄)₂] units. Cohesion of the [TiO(SO₄)₂] units into framework is provided via two crystallographically independent Na atoms. The mineral is named after the Calama commune and the Calama city (IMA2016-036).