Alcaparrosaite, K (sub 3) Ti (super 4+) Fe (super 3+) (SO (sub 4) ) (sub 4) O(H (sub 2) O) (sub 2) , a new hydrophobic Ti (super 4+) sulfate from Alcaparrosa, Chile

Alcaparrosaite, ideally K (sub 3) Ti (super 4+) Fe (super 3+) (SO (sub 4) ) (sub 4) O(H (sub 2) O) (sub 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 (super -3) , respectively. It is optically biaxial (+) with alpha = 1.643(1), beta = 1.655(1), gamma = 1.680(1) (white light), 2V (sub meas) = 70(2) degrees and 2V (sub calc) = 70.3 degrees . The mineral exhibits strong parallel dispersion, r < v. The optical orientation is X = b; Y^c = 27 degrees in the obtuse angle beta . No pleochroism was observed. Electron-microprobe analyses (average of 4) provided: Na (sub 2) O 0.32, K (sub 2) O 20.44, Fe (sub 2) O (sub 3) 11.58, TiO (sub 2) 11.77, P (sub 2) O (sub 5) 0.55, SO (sub 3) 47.52, H (sub 2) O 5.79 (calc); total 97.97 wt.%. The empirical formula (based on 19 O) is (K (sub 2.89) Na (sub 0.07) ) (sub Sigma 2.96) TiO(SO (sub 4) ) (super 2-) (sub 2) (S (sub 0.99) P (sub 0.01) O (sub 4) ) (sub 4) O (sub 0.72) (OH) (sub 0.28) (H (sub 2) O) (sub 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) Aa, beta = 94.076(7) degrees , V = 1542.01(12) Aa (super 3) and Z = 4. The eight strongest lines in the X-ray powder diffraction pattern [d (sub obs) in Aa (I (sub rel) ) (hkl)] are 6.907 (41)(021,110); 3.628 (34) (023,1i13); 3.320 (32) (21i02); 3.096 (100) (202,1i33,150); 3.000 (40) (1i51);> 2.704 (38) (2i23,152); 1.9283 (30) (1i55); 1.8406 (31) (3i53,206). In the structure of alcaparrosaite (R (sub 1) = 2.57% for 1725 F (sub o) > 4Sigma F), Ti (super 4+) and Fe (super 3+) , in roughly equal amounts, occupy the same octahedrally coordinated site. Octahedra are linked into dimers by corner sharing. The SO (sub 4) 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.