Abstract

Columbite-group minerals and ixiolite with extremely variable concentrations of scandium are widespread in moderately to highly fractionated rare-element granitic pegmatites. Columbite-group minerals with 1-3 wt.% Sc 2 O 3 are referred to as scandian columbite-tantalite and show degrees of structural order similar to columbite-tantalite lacking Sc. Disordered structures remain orthorhombic and become ordered upon heating. Pseudo-orthorhombic stannian (Sn-rich), titanian (Ti) and wolframian (W) variants of ixiolite may contain as much as 3.7 wt.% Sc 2 O 3 and revert to monoclinic phases upon heating. Similarly, scandian ixiolite that contains Sc in excess of 3.0 and up to 18.8 wt.% Sc 2 O 3 converts from orthorhombic to monoclinic symmetry upon heating and has a stoichiometry that approaches Sc(Nb, Ta)O 4 . Scandium-bearing columbite-tantalite and ixiolite show similar ranges in Mn/(Mn+Fe) and Ta/(Ta+Nb) values, but noticeably different Sn, Ti and Sc contents. Scandium is incorporated into the columbite and ixiolite structures via the coupled substitution Sc (super 3+) +(Ti,Sn) (super 4+) = (Fe,Mn) (super 2+) +(Nb,Ta) (super 5+) ; it is strongly partitioned into the (Fe,Mn) site, whereas the Ti and Sn prefer the (Nb,Ta) site. Within the (Fe,Mn) site, the substitution of Sc for Fe is more prevalent. In most pegmatites, Sc fractionation in columbite-tantalite, stannian ixiolite, titanian ixiolite and wolframian ixiolite is erratic, unlike scandian ixiolite, which shows strong enrichment in Sc with increasing Mn, Ta and Sn.

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