• journal_title：Mineralogical Magazine
• Contributor：A. R. Kampf ; S. J. Mills ; R. M. Housley ; P. A. Williams ; M. Dini
• Publisher：Mineralogical Society of Great Britain and Ireland
• Date：2012-
• Format：text/html
• Language：en
• Identifier：10.1180/minmag.2012.076.4.03
• journal_abbrev：Mineralogical Magazine
• issn：0026-461X
• volume：76
• issue：4
• firstpage：851
• section：Articles

Alcaparrosaite, ideally K₃Ti⁴⁺Fe³⁺ (SO₄)₄O(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 α = 1.643(1), β = 1.655(1), γ = 1.680(1) (white light), 2V_meas = 70(2)° and 2V_calc = 70.3°. The mineral exhibits strong parallel dispersion, r < v. The optical orientation is X = b; Y^c = 27° in the obtuse angle β. 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 (K_2.89Na_0.07)_Σ2.96 (S_0.99P_0.01O₄)₄O_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) Å, β = 94.076(7)°, V = 1542.01(12) ų and Z = 4. The eight strongest lines in the X-ray powder diffraction pattern [d_obs in Å (I_rel) (hkl)] are 6.907 (41)(021,110); 3.628 (34) (023,1İ13); 3.320 (32) (21İ02); 3.096 (100) (202,1İ33,150); 3.000 (40) (1İ51);> 2.704 (38) (2İ23,152); 1.9283 (30) (1İ55); 1.8406 (31) (3İ53,206). In the structure of alcaparrosaite (R₁ = 2.57% for 1725 F_o > 4ΣF), 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.