Bismutotantalite from Northwestern Argentina: Description and crystal structure

GALLISKI M. A. MÁRQUEZ-ZAVALÍA M. F. COOPER, M. CERNÝ P. Y HAWTHORNE F.

La Plata

Congreso; 5º Congreso de Mineralogía y Metalogenia; 2000

INREMI

Bismutotantalite from La Elvirita, a highly evolved rare-element granitic pegmatite placedin Salta, Northwestern Argentina is studied. The mineral occurs in the albite-rich core of thepegmatite associated preferentially with bismuth, bismuthinite, tapiolite, manganotantalite,microlite, uranmicrolite, bismutomicrolite, hafrxian zircon and montebrasite. A fresh, cm-sizedcrystal, with dark gray color, greasy luster and G = 8.809 gr/cm 3 was studied. In polished section itis gray with very weak birreflectance and shows two phases. EPMA analysis of both give averagesof [13i] (Bi098Sbo.02)(Tao 89Nbo 1)04 and [Bi,Sb] = (Bio.68Sho.32)(Tao.s9Nbo.11)04 respectively.Refined powder-diffraction data of [Bi] give a 4.968(1), b 11.796(3), c 5.646(1) Á, V= 330.85(9)A3 . The crystal structure of [Bi] and [Bi,Sb] were solved by direct methods to an R index of 1.9%and 2.4% based on 450 and 434 observed (4. ) reflections respectively, measured with MoK Xradiation.Both phases are orthorhombic, space group Pcnn, Z = 4, and the results for [Bu] are a4.9652(4), b 11.7831(16), c 5.6462(5) (A), V 330.32(6) (A3), and for [Bi,Sb] are a 4.9471(4), b11.7878(7), c 5.6048(3) (A), V 326.83(4) (A3). These results show that bismutotantalite canaccommodate up to —40% Sb3+ in the place of Bi3+ without changing to the Pc2in polar structure ofstibiotantalite.494