Original paper

Yanomamite, InAsO4.2H2O, a new indium mineral from topaz-bearing greisen in the Goiás Tin Province, Brazil

Botelho, Nilson F.; Roger, Guy; d'Yvoire, Ferdinand; Moelo, Yves; Volfinger, Marcel

European Journal of Mineralogy Volume 6 Number 2 (1994), p. 245 - 254

35 references

published: Mar 31, 1994
manuscript accepted: Dec 16, 1993
manuscript received: Jul 23, 1993

DOI: 10.1127/ejm/6/2/0245

BibTeX file

ArtNo. ESP147050602008, Price: 29.00 €

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Abstract

Abstract Yanomamite occurs as green to yellow-green idiomorphic crystals in aggregates associated with scorodite in quartz-topaz greisen in the tin deposit of Mangabeira, Goias State, Brazil. Yanomamite crystals are always coated by a thin film of In-rich scorodite. Microprobe analyses give (wt. %): In2O3 45.8(3), Fe2O3 1.1(2), AI2O3 0.21(2), AS2O5 40.1 (2), H2O 12.78 by difference, with the structural formula (In0.94Fe0.04Al0.01)Asθ4 . 2.02 H2O Yanomamite is isotypic with scorodite, and belongs to the variscite series. The mineral is orthorhombic, space group Pbca, with a = 10.446(6), b = 9.085(4), c = 10.345(6) A, V= 981.8(7) Å3, Z = 8 ; the synthetic Fe-freeocompound has a = 10.471(3), b = 9.092(2), c = 10.341(2) A, V= 984.5(5) Å3. The strongest reflections [d in A, (I/Io), (hkl)] of the powder diffraction pattern of natural yanomamite are: 5.70 (70)(111), 4.53 (100)(020,210), 4.163 (50)(021), 3.874 (60)(121), 3.250 (60)(122), 3.110 (50)(311), 2.656 (40)(123), 2.541 (40)(231). Dcaic = 3.876(3) g/cm*; VHN25 = 631 kg/mm2 ; 2VZ (meas.) = 55-76°. The average refractive index is n = 1.65. The unit-cell volume versus ionic radius of the tri- or divalent cations shows a very good correlation in the variscite isotypic series (13 arsenates, phosphates and selenates). Yanomamite and In-bearing scorodite are cogenetic, formed by alteration of early arsenopyrite and In-rich sphalerite. This replacement may be related to a hydrothermal process coeval with the main cassiterite deposition, rather than to supergene alteration.

Keywords

yanomamiteindiumarsenateBrazilnew mineralvariscite series