Exsolution and breakdown of scandian and tungstenian Nb-Ta-Ti-Fe-Mn phasesin niobian rutile

Niobian rutile commonly exsolves titanian ferrocolumbite to manganocolumbit e or ixiolite with elevated contents of Nb, Mn and Sc, W or Sn, but Further breakdown of the exsolved phase has nor been reported to date. Niobian rut ile from Eptevann. southern Norway, exsolved titanian-scandian phase El of the A(3+)BO(4) type, which subsequently broke down to a (Sc,Mn)-enriched an d U-depleted phase E2(11 to 12.8 wt.% Sc2O3), a Ti-rich bur (Sc,Mn,U)-poor phase E3, and a (U,Y)-rich but (Ti,Sc,Mn)-depleted, metamict and hydrous ph ase X (2.5 to 3 wt.% UO2). Niobian rutile from Ilmen Mountains, Russia, exs olved a (Ti,U)poor. Mn-rich tungstenian ixiolite (10 to 14 wt.% WO3), which subsequently broke down into a Ti-poor but W-rich phase (similar to 30 wt. % WO2) and a U-enriched but W-depleted mineral (similar to6 wt.% WO3, 0.7 t o 1.5 wt.% UO2). The second-stags breakdown of the exsolved phases reflects the metastable nature of these minerals. which feature disordered populati ons of divalent to hexavalent cations in an exclusively octahedral array of oxygen anions, with consequent long-range disorder and local imbalances of electrostatic charges. Under low-temperature conditions, insufficient for further exsolution and ordering, the exsolved phases readily respond to the action of aqueous fluids by breakdown to more cation-selective phases. How ever, incomplete separation of differ ent categories of cations and non-int egral stoichiometries of the products suggest that they did not attain equi librium.