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_2mm combined in bunches or radial spherulitic clusters up to 4mm across; rarely prismatic crystals up to 1_1_3mm 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. Dcalc is 2.45 g · cm_3. Calamaite is optically biaxial (þ), a = 1.557(2), b = 1.562(2), g = 1.671(3), 2Vmeas=30(10)°. The IR spectrum is reported. The chemical composition (wt%, electron microprobe data; H2O content is calculated for 2H ₂O pfu) is: Na ₂O 18.21,K ₂O 0.06, Fe ₂O ₃ 1.58, TiO ₂ 21.80, SO ₃ 48.25, H2Ocalc 10.74, total 100.73. The empirical formula calculated on the basis of 11 O apfu is: Na1:97ðTi0:92Fe3þ 0:07 ÞS0:99S2:02O9·2H2O Calamaite is orthorhombic, Ibam, a=16.0989(11), b=16.2399(9), c=7.0135(4)A, V=1833.6 (2)A _ 3, and Z = 8. The strongest reflections of the powder XRD pattern [d,A _(I)(hkl)] are: 8.10(1 0 0)(0 2 0, 2 0 0), 5.04(55)(1 2 1, 2 11), 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 TiO ₆ octahedra are interconnected via O vertices to form infinite (Ti–O)∝ chains. The remaining vertices of each TiO ₆ octahedron are shared with SO ₄ tetrahedra thus forming one-dimensional [TiO(SO ₄) ₂] units. Cohesion of the [TiO(SO ₄) 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).