Cerny and Ercita reviewed in 1985 the current understanding of the chemistry and structural properties is provided for columbite-tantalite, ferrotapiolite-manganotapiolite, ixiolite, wodginite, tantalian and niobian rutile, tantalian-niobian cassiterite, and the pyrochlore group. Understanding of the crystal chemistry of some of the most common species appears to be rather spurious. Paragenetic assemblages of primary Nb, Ta-bearing minerals typical of different classes of rare-element pegmatites are summarized. They show extensive gaps in upgrading old and partly obsolete information. In contrast, a review of recently discovered "exotic" Nb, Ta-bearing minerals is complemented by a summary of late alterations and replacements which is well documented.
Extensive variety in fractionation of columbite-tantalite is demonstrated on some pegmatites, pegmatite groups and fields. More data from different pegmatite types are required, in conjunction with experimental work, to understand the factors regulating the course of fractionation. Compositional and structural stabilities are poorly understood. They require thorough experimental investigation, under conditions comparable to those of crystallizing pegmatite melts and fluids.
The authors concluded that despite the recent progress in our understanding of Nb,Ta-bearing minerals in granitic pegmatites, and partly because of this progress, many fundamental problems remain unresolved, ignorance, five principal approaches should be followed:
i) paragenetic, compositional and quantitative structural documentation of all members of natural Nb,Ta-mineral sequences from geologically interpreted and classified pegmatites, pegmatite groups and fields, to improve the existing data base;
(ii) structure refinements of all new minerals, and systematic structure refinements in all compositional and/or order-disorder series and groups, to understand the crystallochemical state and potential of individual species and their relationships;
(iii) experimental definition of compositional and structural stability relationships under geologically realistic conditions, to interpret the natural assemblages of Nb,Ta-bearing species ;
(iv) geochemical studies of the assemblages and associations carrying Nb, Ta-minerals, in terms of bulk composition, trace element distributions, stable and radiogenic isotope fractionation, and fluid inclusion characteristics, to establish constraints on their mode(s) of formation ; and
(v) experimental modelling of Nb,Ta migration and precipitation in granitic melts, co-existing supercritical fluids and post-magmatic solutions,to verify the interpretation of natural fractionation products.
The above program is a tall order indeed, but most of its components are already pursued in many institutions around the world, and substantial advances in our knowledge of Nb,Ta mineralogy should be forthcoming in the foreseeable future.
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