Bismutotantalite from Northwestern Argentina: Description and Crystal Structure.

GALLISKI, M. A., MÁRQUEZ ZAVALÍA, M. F., COOPER, M. A., ČERNÝ, P. Y HAWTHORNE, F. C.

CANADIAN MINERALOGIST

Mineralogical Association of Canada

Lugar: Ottawa; Año: 2001 vol. 39 p. 103 - 110

0008-4476

Bismutotantalite occurs in albite-rich cores of the La Elvirita granitic pegmatites, northwestern Argentina, associated mainlywith bismuth, bismuthinite, ferrotapiolite, manganotantalite, microlite, uranmicrolite, bismutomicrolite, hafnian zircon andmontebrasite. A fresh, cm-sized crystal, dark grey with a greasy luster and D = 8.809 g/cm3, was examined. In reflected light, itis grey with very weak bireflectance; two phases can be distinguished. Electron-microprobe analysis gives the host bismutotantalite[Bi] as (Bi0.98Sb0.02)(Ta0.89Nb0.11)O4; the subsidiary phase [Bi,Sb], (Bi0.68Sb0.32)(Ta0.89Nb0.11)O4, is enriched in Sb. Least-squaresrefinement of X-ray powder-diffraction data of [Bi] gave a 4.968(1), b 11.796(3), c 5.646(1) Å , V = 330.85(9) Å3. The crystalstructures of [Bi] and [Bi,Sb] were refined to R indices of 1.9 and 2.4%, based on 387 and 377 observed (4) reflections,respectively, measured with MoK X-radiation. Both phases are orthorhombic, space group Pcnn, Z = 4; [Bi] has a 4.9652(4), b11.7831(16), c 5.6462(5) Å, V 330.32(6) Å3, and [Bi,Sb] has a 4.9471(4), b 11.7878(7), c 5.6048(3) Å, V 326.83(4) Å3. Theseresults show that the centrosymmetric structure of bismutotantalite can accommodate up to ~40% Sb3+ substituting for Bi3+without changing to the Pc21n structure of stibiotantalite.Keywords: bismutotantalite, crystal-structure refinement, electron-microprobe analysis, granitic pegmatite, La Elvirita, Argentina.