Ammoniotinsleyite, (nh4)al2(po4)2(oh)⋅2h2o, a New Mineral Species from the Guano Deposit at Pabellón De Pica, Iquique Province, Chile

The new leucophosphite-group mineral ammoniotinsleyite is found in a guano deposit located on the PabellOn de Pica Mountain, Iquique Province, Tarapaca Region, Chile. Associated minerals are halite, gypsum, salammoniac and clay minerals. Ammoniotinsleyite occurs as pink to pale violet globular aggregates up to 3 mm across with individual single crystals similar to 10-15 mu m. The mineral is brittle. Its Mohs hardness is 4. D-meas. = 2.42(2) g cm(-3) and D-calc = 2.451 g cm(-3). The IR spectrum shows the presence of NH4+ and PO43- groups and H2O molecules. Ammoniotinsleyite is optically biaxial (+), alpha = L557(2), beta = 1.559 (calc.), gamma = 1.563(2) (lambda = 589 nm); and 2V(meas). = 75(10)degrees. The chemical composition (K, Mg, Ca, Al, Fe and P from electron-microprobe data; H, C and N measured by gas chromatography on products of ignition at 1200 degrees C; wt.%) is: (NH4)(2)O 7.25, K2O 1.50, MgO 0.42, CaO 0.34, Al2O3 29.91, Fe2O3 2.36, P2O5 43.97, H2O 14.89, CO2 below detection limit, total 100.64. The empirical formula is [(NH4)(0.88)K0.10Ca0.02)](Sigma 1.00)(Al1.86Fe0.093+Mg0.03)(Sigma 1.98)(PO4)(1.96)(OH)(1.05)center dot 2.11H(2)O. The idealised formula is (NH4)(2)Al-2(PO4)(2)(OH)center dot 2H(2)O. The crystal structure of ammoniotinsleyite was refined based on powder X-ray diffraction data, using the Rietveld method. The final agreement factors are: R-P = 0.0071, R-wp = 0.0093 and R-obs = 0.0167. The new mineral is isostructural with tinsleyite, spheniscidite and leucophosphite. It is monoclinic, space group P2(1/n), a = 9.5871(1) angstrom, b = 9.6089(1) angstrom, c = 9.6467(2) angstrom, beta= 103.4461(8)degrees, V = 864.31(2) angstrom(3) and Z = 4. The strongest reflections of the powder X-ray diffraction pattern [d,angstrom(1,%)(hkl)] are: 7.56(23)(101), 6.71(79)(011, 110), 5.947 (100)(101, 111), 4.676(36)(002, 200), 3.032(28)(113, 031, 130), 2.958(25)(222, 310, 131) and 2.635(29)(231).