Two-stage exsolution of a titanian (Sc,Fe3+)(Nb,Ta)O-4 phase in niobian rutile from southern Norway

Niobian rutile from Haverstad and an unspecified locality in central Ivelan d, southern Norway, displays a novel two-stage exsolution of phase E, corre sponding to titanian (Sc,Fe3+)(Nb>Ta)O-4, not yet characterized as a minera l species. In general, rutile retains Ti, Fe3+ (relative to Fe2+), Sigma Fe (relative to Mn) and Ta (relative to Nb), whereas the exsolved phase E con centrates Nb, Ta, Sigma Fe, Sc, W, Zr, Mg and U. In the first stage, phase E forms platelets averaging similar to 20 x 2 mum; their orientation is con trolled by the structure of the host rutile. The platelets contain less tha n or equal to 30.40 wt% TiO2 and Sc2O3 as low as 3.37 wt.%, whereas the rut ile retains less than or equal to 24.00 wt.% Nb2O5, less than or equal to8. 15 wt.% Ta2O5 and less than or equal to1.61 wt.% Sc2O3. In the second stage : coarsening of phase E produces irregular subround blebs 10 to 30 mum in s ize. Exsolution is advanced during this stage, increasing the Sc content of phase E up to 10.06 wt.% Sc2O3 and reducing its Ti content to as low as 11 .90 wt.% TiO2, whereas the rutile matrix is depleted in Nb, Ta and Sc to as low as 21.30, 7.61 and 0.52 wt.% of the respective oxides. The average Fe3 +:Fe2+ ratio of rutile is 3.51, whereas it decreases to 3.18 in platelets o f phase E, and it drops to 2.60 in the blebs of the second-generation phase E. The platelet stage might prove to he typical of exsolution of phase E f rom niobian rutile with high (Sc,Fe)(3+)/Fe2+, in contrast to the much more widespread granular exsolution of titanian ixiolite or titanian ferrocolum bite from Sc-poor niobian rutile with Fe3+/Fe2+ much less than 1.