Kettnerita, clinobisvanita y bismutita de la pegmatita SD-2 (Córdoba): descripción mineralógica y cuantificación de los componentes de una mezcla con el método Rietveld

COLOMBO, FERNANDO; CARBONIO, RAÚL E.; PANNUNZIO MINER, ELISA VICTORIA; LIRA, RAÚL

Buenos Aires

Congreso; VI Congreso de Mineralogía y Metalogenia; 2002

Departamento de Ciencias Geológicas (UBA).

A mixture of bismuthite Bi2(CO3)O2, clinobisvanite Bi(VO4) and kettnerite CaBi(CO3)OF (together with quartz and goethite) was found in the SD-2 granite pegmatite located near Tanti, Punilla Department, Córdoba Province (Argentina). Its geographical coordinates are 31º 21´ 55.3´´ S, 64º 28´ 50.3´´ W. It is a zoned body composed mainly of quartz, microcline, plagioclase, biotite and muscovite, with Mn-rich fluorapatite, columbite-group minerals, spessartine, bertrandite, bismutite, kettnerite, clinobisvanite, goethite, abundant beryl, and very scarce rutile and zircon as accessory minerals. The three species are found intimately mixed with quartz and goethite in an opaque brownish yellow  mass of 4 x 2 x 0.7 cm, with earthy luster and conchoidal to irregular fracture. Kettnerite occurs as subhedral crystals  to 55 mm long embedded in finely granular bismutite. Clinobisvanite looks similar to kettnerite under the SEM but its EDS spectrum shows major Bi and V (with traces of Al and Fe). EDS spot analyses show Bi with traces of Fe and Si (bismutite) and Bi and Ca with traces of Al, Si and Fe (kettnerite). The Fe content is probable due to the goethite, and the Si detected reflects quartz in the mixture. The refined cell parameters of the bismutite, kettnerite and clinobisvanite are (in Ǻ): a = 3.877(2), b = 3.878(2) and c = 13.703(2) (for bismutite), a= 3.804(6), b = 3.799(5) and c = 13.688(8) (for kettnerite) and a = 5.238(9), b = 3.149(8), c = 11.735(8) and b = 90.408(5)º (for clinobisvanite).  A quantification of the components in the mixture was accomplished using the Rietveld method. According to the refinement, the mass is composed of 67(1) % of bismutite, 27.6(6) % of kettnerite, 1.6(2) % of clinobisvanite, 1.4(9)% of goethite and 2.0(9) % of quartz. We want to stress the usefulness of the Rietveld method for the quantification of opaque and microcrystalline mixtures, which could not be done by other conventional methods.