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Alcaparrosaite, ideally K3Ti4+Fe3+(SO4)(4)O(H2O)(2), is a new mineral from the Alcaparrosa mine, Cerritos Bayos, El Loa Province, Antofagasta, Chile (IMA2011-024). The mineral occurs on and intergrown with coquimbite, and is also associated with ferrinatrite, krausite, pertlikite, pyrite, tamarugite and voltaite. It is a relatively early phase which forms during the oxidation of pyritic masses under increasingly arid conditions. Alcaparrosaite crystallizes from hyperacidic solutions in a chemical environment that is consistent with its association with coquimbite. It occurs as pale yellow blades and tapering prisms up to 4 mm in length, flattened on {010} and elongated along [100]. The observed crystal forms are {010}, {110}, {1.13.0} and {021}. The mineral is transparent and has a white streak, vitreous lustre, Mohs hardness of about 4, brittle tenacity, conchoidal fracture and no cleavage. The measured and calculated densities are 2.80(3) and 2.807 g cm(-3), respectively. It is optically biaxial (+) with alpha = 1.643(1), beta = 1.655(1), gamma = 1.680(1) (white light), 2V(meas) = 70(2)degrees and 2V(calc) = 70.3 degrees. The mineral exhibits strong parallel dispersion, r < v. The optical orientation is X = b; Y boolean AND c = 27 degrees in the obtuse angle beta. No pleochroism was observed. Electron-microprobe analyses (average of 4) provided: Na2O 0.32, K2O 20.44, Fe2O3 11.58, TiO2 11.77, P2O5 0.55, SO3 47.52, H2O 5.79 (calc); total 97.97 wt.%. The empirical formula (based on 19 O) is (K2.89Na0.07)(Sigma 2.96)Ti0.984+Fe0.973+(S0.99P0.01O4)(4)O-0.72 (OH)(0.28)(H2O)(2). The mineral is hydrophobic, insoluble in cold and hot water, very slowly soluble in acids and decomposes slowly in bases. Alcaparrosaite is monoclinic, C2/c, with the cell parameters a = 7.55943(14), b = 16.7923(3), c = 12.1783(9) angstrom, beta = 94.076(7)degrees, V = 1542.01(12) angstrom(3) and Z = 4. The eight strongest lines in the X-ray powder diffraction pattern [d(obs) in angstrom (I-rct) (hkl)] are 6.907 (41) (021,110); 3.628 (34) (023,(1) over bar 13); 3.320 (32) ((2) over bar 02); 3.096 (100) (202,(1) over bar 33,150); 3.000 (40) ((1) over bar 51); 2.704 (38) ((2) over bar 23,152); 1.9283 (30) ((1) over bar 55); 1.8406 (31) ((3) over bar 53,(2) over bar 06). In the structure of alcaparrosaite (R-1 = 2.57% for 1725 F-o > 4 sigma F), Ti4+ and Fe3+, in roughly equal amounts, occupy the same octahedrally coordinated site. Octahedra are linked into dimers by corner sharing. The SO4 tetrahedra link the dimers into chains parallel to [001] and link the chains into undulating sheets parallel to {010}. The sheets link via 10- and 11-coordinated K atoms in the interlayer region. The structure shares some features with that of goldichite.