YANOMAMITE, INASO4.2H2O, A NEW INDIUM MINERAL FROM TOPAZ-BEARING GREISEN IN THE GOIAS TIN PROVINCE, BRAZIL

Yanomamite occurs as green to yellow-green idiomorphic crystals in agg regates associated with scorodite in quartz-topaz greisen in the tin d eposit of Mangabeira, Goias State, Brazil. Yanomamite crystals are alw ays coated by a thin film of In-rich scorodite. Microprobe analyses gi ve (wt. %) : In2O3 45.8(3), Fe2O3 1.1(2), Al2O3 0.21(2), AS2O5 40.1 (2 ), H2O 12.78 by difference, with the structural formula (In0.94Fe0.04A l0.01)ASO4.2.02 H2O. Yanomamite is isotypic with scorodite, and belong s to the variscite series. The mineral is orthorhombic, space group Pb ca, with a = 10.446(6), b = 9.085(4), c = 10.345(6) angstrom, V = 981. 8(7) angstrom3, Z = 8 ; the synthetic Fe-free compound has a = 10.471( 3), b = 9.092(2), c = 10.341(2) angstrom, V = 984.5(5) angstrom3. The strongest reflections [d in angstrom, (I/I0), (hkl)] of the powder dif fraction pattern of natural yanomamite are : 5.70 (70)(111), 4.53 (100 )(020,210), 4.163 (50)(021), 3.874 (60)(12t), 3.250 (60)(122), 3.110 ( 50)(311), 2.656 (40)(123), 2.541 (40)(231). D(calc) = 3.876(3) g/cm3 ; VHN25 = 631 kg/mm2 ; 2V(Z) (meas.) = 55-76-degrees. The average refra ctive index is n = 1.65. The unit-cell volume versus ionic radius of t he tri-or divalent cations shows a very good correlation in the varisc ite isotypic series (13 arsenates, phosphates and selenates). Yanomami te and In-bearing scorodite are cogenetic, formed by alteration of ear ly arsenopyrite and In-rich sphalerite. This replacement may be relate d to a hydrothermal process coeval with the main cassiterite depositio n, rather than to supergene alteration.