Ixiolite

Abstract: A systematic review of the oxide minerals of niobium and tantalum is presented, including varietal names, crystal chemistry, structural features, relationships to other phases and paragenesis of each mineral group, series or species. A separate section deals with oxide minerals that occur exclusively as secondary, low-temperature phases of largely metasomatic origin. Oxide minerals of Sn, Ti and W that carry substantial Nb and Ta are discussed as well. A new crystal chemical classification of the oxide minerals of Nb and Ta is presented.

Abstract: The phase relations and microwave dielectric properties of (1−x)ZnNb2O6–xTiO2 were investigated using x-ray powder diffraction and a network analyzer. Four phase regions were studied with increasing TiO2 mol% (x): columbite solid solution, ixiolite (ZnTiNb2O8) solid solution, mixture of ixiolite and rutile solid solutions, and rutile solid solution. It was suggested that the microwave properties depend on crystal structure rather than chemical composition. In the columbite solid solution region, an order–disorder transition was found with an increasing amount of TiO2, and the quality factor decreased sharply. ZnTiNb2O8 (x = 0.5), has a fully disordered structure and possesses a quality factor of 42,500, relative dielectric constant (er) of 34.3, and temperature coefficient of resonant frequency (τf) of −52 ppm/°C. In the mixture region of ixiolite and rutile structure, τf was modified to around 0 ppm/°C.

Abstract: The Borborema Pegmatitic Province (BPP), northeastern Brazil, is historically important for tantalum mining and also famous for top-quality specimens of exotic Nb–Ta oxides and, more recently, for the production of gem quality, turquoise blue, ‘Paraiba Elbaite.’ With more than 750 registered mineralized rare-element granitic pegmatites, the BPP extends over an area of about 75 by 150 km in the eastern part of the Neoproterozoic Serido Belt. The Late Cambrian pegmatites are mostly hosted by a sequence of Neoproterozoic cordierite–sillimanite biotite schists of the Serido Formation and quartzites and metaconglomerates of the Equador Formation. The trace-element ratios in feldspar and micas allow to classify most pegmatites as belonging to the beryl–columbite phosphate subtype. Electron microprobe analyses (EMPA) of columbite, tapiolite, niobian–tantalian rutile, ixiolite and wodginite group minerals from 28 pegmatites in the BPP are used to evaluate the effectiveness of Nb–Ta oxide chemistry as a possible exploration tool, to trace the degree of pegmatite fractionation and to classify the pegmatites. The columbite group mineral composition allows to establish a compositional trend from manganoan ferrocolumbite to manganocolumbite and on to manganotantalite. This trend is typical of complex spodumene- and/or lepidolite-subtype pegmatites. It clearly contrasts with another trend, from ferrocolumbite through ferrotantalite to ferrowodginite and ferrotapiolite compositions, typical of pegmatites of the beryl–columbite phosphate subtype. Large scatter and anomalous trends in zoned crystals partially overlap and conceal the two main evolution patterns. This indicates that a large representative data set of heavy mineral concentrate samples, collected systematically along cross-sections, would be necessary to predict the metallogenetic potential of individual pegmatites. Other mineral species, e.g. garnets and/or tourmaline, with a more regular distribution than Nb–Ta oxides, would be more appropriate and less expensive for routine exploration purposes. The currently available Nb–Ta oxide chemistry data suggest the potential for highly fractionated Ta–Li–Cs pegmatites in the BPP, so far undiscovered, and encourages further, more detailed research.