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UNITED STATES MINERAL RESOURCES 



of niobium and tantalum. Where present in many 

 rock-forming minerals, the niobium and tantalum 

 are in limited substitution for titanium, tin, tungs- 

 ten, and zirconium. 



MINERALS 



A glossary of niobium and tantalum minerals 

 given by Parker and Fleischer (1968) shows that 

 niobium and tantalum minerals are chiefly oxides, 

 multiple oxides, and hydroxides, as well as a few 

 silicates and one borate. More than 90 species are 

 known; the most important ore minerals and their 

 chemical compositions are listed below: 



Columbite- 



tantalite (Fe,Mn) (Nb,Ta).0, 



Euxenite (Y,Ca,Ce,U,Th) (Nb,Ti,Ta):0„ 



Fergusonite (Y,Er,Ce,Fe) (Nb,Ta,Ti)0. 



Ixiolite (Ta,Nb,Sn,Fe,Mn)i08 



Loparite 



(variety of 



perovskite) — (Ce,Na,Ca)»(Ti,Nb)oOe 



Microlite (Na,Ca)2Ta2(0,0H,F), 



Murmanite Na.(Ti,Nb)= Si.C-wH.O 



Pandaite (Ba.Sr) (Nb.Ti) (0,OH)t 



Pyrochlore (Na,Ca)2Nb2(0,0H,F), 



Samarskite (Y,Er,Ce,U,Ca,Fe,Pb,Th) (Nb.Ta, Ti,Sn)206 



Wodginite (Ta,Nb,Sn,Mn,Fe)i6032 



TYPES OF DEPOSITS 



Ore concentrations of niobium and tantalum occur 

 in certain types of magmatic rocks and endogenic 

 derivatives of those rocks. Commercial deposits have 

 been found in various related rocks of alkalic com- 

 plexes, such as nepheline syenites and carbonatites, 

 certain granites, pegmatites, and placers. Some low- 

 grade, widely disseminated, unexploited deposits of 

 niobium occur in riebeckite granite and in bauxite 

 derived from nepheline syenite. 



ALKALIC ROCK COMPLEXES 



NEPHELINE SYENITES 



Nepheline-bearing syenites are among the types 

 of rocks that may be greatly enriched in niobium 

 and tantalum, particularly in their late phases such 

 as pegmatites, albitized zones, and some carbona- 

 tites. Nepheline syenites are found in alkalic rock 

 provinces throughout the world but are best known 

 from their many occurrences in the U.S.S.R. where, 

 according to Es'kova (1960), they constitute 1 per- 

 cent of the exposed igneous rocks. In eight of the 

 prominent nepheline syenite massifs of the U.S.S.R., 

 the average niobium content ranges from 100 to 900 

 ppm, and the average tantalum content ranges from 

 12.0 to 20.5 ppm (Parker and Fleischer, 1968) . 



On the basis of chemical and mineralogical dif- 

 ferences, there appear to be two distinct types of 

 nepheline syenites, which have been given the dis- 



tinguishing terms "agpaitic" and "miaskitic." As 

 originally proposed by Ussing (1911), the agpaitic 

 type is characterized by an excess of alkalies 

 (K20+Na20) over alumina, in contrast to the "com- 

 mon" type in which this ratio is 1 or less. The 

 agpaitic type is also characterized by the presence 

 of complex zirconium- and chlorine-bearing minerals 

 and sodic amphiboles and pyroxenes and by the 

 general absence of calcite. Because of their enrich- 

 ment in typical "rest elements" and in volatiles 

 such as F, CI, and H2O, agpaitic nepheline syenites 

 have a compositional resemblance to pegmatites 

 (Sorensen, 1960). The "common" or miaskitic nephe- 

 line syenite, in addition to having the lower alkali- 

 alumina ratio, differs from the agpaitic type in 

 mineral assemblage by containing more of the com- 

 mon rock-forming species, such as biotite, augite, 

 hornblende, sphene, and apatite, and fewer of the 

 rare minerals of complex chemistry that are found 

 in the agpaitic type. These and other characteristics 

 of the two types, as well as some notable occur- 

 rences, are described by Heinrich (1966, p. 19). 



Although both agpaitic and miaskitic syenites may 

 be enriched in niobium and tantalum, they differ in 

 the kind and number of minerals in which these 

 elements occur. In agpaitic syenites and their deriva- 

 tives, these two elements are largely fixed in titan- 

 ium and zirconium minerals, many of which are rare 

 species (Es'kova, 1960) . In the miaskitic type, some 

 of these minerals may be present, but there is a 

 greater tendency for the formation of independent 

 niobium-tantalum minerals such as pyrochlore, par- 

 ticularly in the late magmatic and postmagmatic 

 derivatives. 



Niobium- and tantalum-bearing agpaitic rocks 

 make up much of the great Lovozero alkali massif 

 in the Kola Peninsula, U.S.S.R., and have been 

 described in detail by Vlasov, Kuz'menko, and Es'- 

 kova (1959). The massif as a whole contains about 

 900 ppm niobium and 70 ppm tantalum, but the con- 

 tent of these elements varies not only among the 

 various differentiates but also within the layered 

 rock units themselves, as for example, in porphyritic 

 lujarvrite, which may contain 0.03-0.92 percent 

 (Nb,Ta)205. 



The principal niobium mineral in the Lovozero 

 complex is loparite, a member of the perovskite 

 group which contains about 12 percent (Nb,Ta)205. 

 It is found to some extent in all the rocks of the 

 complex but is most abundant where differentiation 

 has been greatest, particularly in the base of urtite 

 (nepheline-rich syenite) layers and the top of the 

 underlying trachytic nepheline syenite. Niobium and 

 tantalum are major constituents of several other 



