ALBERT

Description: Tapiaite (IMA2014-024), Ca 5 Al 2 (AsO 4 ) 4 (OH) 4 ·12H 2 O, is a new mineral from the Jote mine, Tierra Amarilla, Copiapó Province, Atacama, Chile. The mineral is a late-stage, low-temperature, secondary mineral occurring with conichalcite, joteite, mansfieldite, pharmacoalumite, pharmacosiderite and scorodite in narrow seams and vughs in the oxidized upper portion of a hydrothermal sulfide vein hosted by volcanoclastic rocks. Crystals occur as colourless blades, flattened on {101I} and elongated and striated along [010], up to ~0.5 mm long, and exhibiting the forms {101I}, {101} and {111}. The blades are commonly intergrown in subparallel bundles and less commonly in sprays. The mineral is transparent and has a white streak and vitreous lustre. The Mohs hardness is estimated to be between 2 and 3, the tenacity is brittle, and the fracture is splintery. It has two perfect cleavages on {101} and {101I}. The calculated density based on the empirical formula is 2.681 g cm –3 . It is optically biaxial (+) with α = 1.579(1), β = 1.588(1), = 1.610(1) (white light), 2V meas = 66(2)° and 2V calc = 66°. The mineral exhibits no dispersion. The optical orientation is X [101I]; Y = b, Z [101]. The electron-microprobe analyses (average of five) provided: Na 2 O 0.09, CaO 24.96, CuO 0.73, Al 2 O 3 10.08, Fe 2 O 3 0.19, As 2 O 5 40.98, Sb 2 O 5 0.09, H 2 O 23.46 (structure), total 100.58 wt.%. In terms of the structure, the empirical formula (based on 32 O a.p.f.u.) is (Ca 4.83 $${\mathrm{Cu}}_{0.10}^{2+}$$ Na 0.03 ) 4.96 (Al 2.14 $${\mathrm{Fe}}_{0.03}^{3+}$$ ) 2.17 [( $${\mathrm{As}}_{3.87}^{5+}$$ $${\mathrm{Sb}}_{0.01}^{5+}$$ ) 3.88 O 16 ][(OH) 3.76 (H 2 O) 0.24 ] 4 (H 2 O) 10 ·2H 2 O. The mineral is easily soluble in RT dilute HCl. Tapiaite is monoclinic, P 2 1 / n , with unit-cell parameters a = 16.016(1), b = 5.7781(3), c = 16.341(1) Å, β = 116.704(8)°, V = 1350.9(2) Å 3 and Z = 2. The eight strongest lines in the powder X-ray diffraction pattern are [ d obs Å ( I )( hkl )]: 13.91(100)(1I01), 7.23(17)(200,002), 5.39(22)(110,011), 4.64(33)(1I12,2I11,3I03), 3.952(42)(1I13,3I11,2I13), 3.290(35)(2I14,4I12,1I14,4I11), 2.823(39)(303,3I15) and 2.753(15)(5I13,1I15,121,5I11). The structure of tapiaite ( R 1 = 5.37% for 1733 F o 〉 4 F ) contains Al(AsO 4 )(OH) 2 chains of octahedra and tetrahedra that are topologically identical to the chain in the structure of linarite. CaO 8 polyhedra condense to the chains, forming columns, which are decorated with additional peripheral AsO 4 tetrahedra. The CaO 8 polyhedra in adjacent columns link to one another by corner-sharing to form thick layers parallel to {101I} and the peripheral AsO 4 tetrahedra link to CaO 6 octahedra in the interlayer region, resulting in a framework structure.