Bismutotantalite from northwestern Argentina: Description and crystal structure

Bismutotantalite occurs in albite-rich cores of the La Elvirita granitic pe gmatites, northwestern Argentina, associated mainly with bismuth, bismuthin ite. ferrotapiolite, manganotantalite, microlite, uranmicrolite, bismutomic rolite. hafnian zircon and montebrasite. A fresh, cm-sized crystal, dark gr ey with a greasy luster and D = 8.809 g/cm(3), was examined. In reflected l ight, it is grey with very weak bireflectance; two phases can br distinguis hed. Electron-microprobe analysis gives the host bismutotantalite [Bi] as ( Bio(0.98)Sb(0.02))(Ta0.89Nb0.11)O-4: the subsidiary phase [Bi,Sb], (Bi0.68S b0.32)(Ta0.89Nb0.11)O-4, is enriched in Sb. Least-squares refinement of X-r ay powder-diffraction data of [Bi] gave a 4.968(1). b 11.796(3), c 5.646(1) Angstrom, V= 330.85(9) Angstrom (3). The crystal structures of [Bi] and [B i,Sb] were refined to R indices of 1.9 and 2.4%, based on 387 and 377 obser ved (4 sigma) reflections, respectively, measured with MoK alpha X-radiatio n. Both phases are orthorhombic. space group Pcnn, Z= 4: [Bi] has a 4.9652( 4), h 11.7831(16), c 5.6462(5) Angstrom, V 330.32(6) Angstrom (3), and [Bi, Sb] has a 4.9471(4), b 11.7878(7), c 5.6048(3) Angstrom, V 326.83(4) Angstr om (3). These results show that the centrosymmetric structure of bismutotan talite can accommodate up to similar to 40% Sb3+ substituting for Bi3+ with out changing to the Pc2(1)n structure of stibiotantalite.