EXSOLUTION AND BREAKDOWN OF SCANDIAN AND TUNGSTENIAN Nb Ta Ti Fe Mn PHASES IN NIOBIAN RUTILE

Abstract

Niobian rutile commonly exsolves titanian ferrocolumbite to manganocolumbite or ixiolite with elevated contents of Nb, Mn and Sc, W or Sn, but further breakdown of the exsolved phase has not been reported to date. Niobian rutile from Eptevann, southern Norway, exsolved titanian-scandian phase E1 of the A 3+ B O 4 type, which subsequently broke down to a (Sc,Mn)-enriched and U-depleted phase E2 (11 to 12.8 wt.% Sc 2 O 3 ), a Ti-rich but (Sc,Mn,U)-poor phase E3, and a (U,Y)-rich but (Ti,Sc,Mn)-depleted, metamict and hydrous phase X (2.5 to 3 wt.% UO 2 ). Niobian rutile from Ilmen Mountains, Russia, exsolved a (Ti,U)-poor, Mn-rich tungstenian ixiolite (10 to 14 wt.% WO 3 ), which subsequently broke down into a Ti-poor but W-rich phase (~30 wt.% WO 3 ) and a U-enriched but W-depleted mineral (~6 wt.% WO 3 , 0.7 to 1.5 wt.% UO 2 ). The second-stage breakdown of the exsolved phases reflects the metastable nature of these minerals, which feature disordered populations 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. However, incomplete separation of different categories of cations and non-integral stoichiometries of the products suggest that they did not attain equilibrium.