Niobian rutile from the McGuire granitic pegmatite, Park County, Colorado:Solid solution, exsolution, and oxidation

Coarse crystals of niobian rutile occur in the hydrothermally altered core- margin zone of the McGuire granitic pegmatite, Park County, Colorado, assoc iated with potassium feldspar, quartz, biotite, ilmenite, and monazite-(Ce) . Primary homogeneous niobian rutile, with Fe3+ congruent to Fe2+ and a sma ll excess of (Fe, Mn) over the amount required to compensate the incorporat ion of (Nb,Ta,W), underwent three stages of exsolution. Primary homogeneous niobian rutile exsolved a fine trellis-like pattern of minor lamellar Nb-b earing pseudorutile I. Most of this phase was broken down to pseudomorphs c onsisting of microgranular Nb-rich pseudorutile II imbedded in niobian-ferr ian "ferropseudobrookite." Continued exsolution in niobian rutile and recon stitution of the early exsolution products generated (Fe, Nb)-depleted, mic rogranular niobian rutile, titanian ferrocolumbite, and minor ilmenite. The se three phases did not attain chemical equilibrium but may represent a sta ble phase assemblage. All these processes seem to have maintained charge ba lance, suggesting a closed system. Subsequent to the three stages of exsolu tion, extensive oxidation converted the mineral assemblages to anatase + he matite + titanian-tungstenian ixiolite; primary ilmenite was oxidized into an anatase + hematite intergrowth. In both cases, the hematite component wa s almost completely leached out, leaving highly porous aggregates of the ot her phases. The exsolution products in niobian rutile are controlled by the (Fe+Mn+Sc)/(Nb+Ta+W) ratio of the primary phase and by its (Fe3++Sc)/(Fe2+Mn) ratio. Dominance of divalent A-cations facilitates exsolution of titan ian ferro- to manganocolumbite or titanian ixiolite, whereas dominant triva lent cations lead to exsolution of titanian (Fe, Sc)3+NbO4 phases. Excess ( Fe, Mn) over the columbite-type Fe2+Nb2 stoichiometry causes exsolution of (Fe, Mn)-rich Nb, Ta-oxide minerals or complementary (Fe, Mn, Ti) phases.