TITANIAN IXIOLITE - NIOBIAN RUTILE INTERGROWTHS FROM THE BORBOREMA PEGMATITIC PROVINCE, NORTHEASTERN BRAZIL

Abstract

Intergrowths of orthorhombic and tetragonal Nb–Ta–Ti–(Sn) oxides were at first recognized by conventional X-ray diffraction and electron-microprobe analyses in six pegmatite occurrences of the Borborema Pegmatitic Province in northeastern Brazil. In three of these cases, electron-diffraction patterns (HRTEM) on chips 0.15 μm thick obtained by focused ion-beam (FIB) sampling were necessary to confirm the preliminary identification of titanian ixiolite. Electron-microprobe analyses reveal very variable bulk-compositions and extents of compositional gaps between the coexisting tetragonal and orthorhombic phases in the Nb,Ta – Ti,Sn – Fe,Mn triangular plot. This behavior is difficult to explain as a result of a simple solvus along a pseudobinary section between rutile and tapiolite – columbite – ixiolite – wodginite. In two of six cases, the data obtained plot exactly within the compositional gaps observed between these phases in other pegmatitic provinces. This apparent violation of the gap is due to abnormally high Ti-contents in ixiolite and high levels of Ta + Nb (up to 55 wt.% Nb + Ta oxides) observed in the tetragonal phase. Therefore, in the cases studied, the observed trends and gaps seem to be the result of disequilibrium (or incomplete equilibrium) between the coexisting phases, despite the exsolution-like, cuneiform to vermicular intergrowths observed in three of the occurrences studied. Compositional growth-zoning observed in the other cases agrees with the hypothesis of disequilibrium. In five of the six cases, a partition of Ta and Fe in favor of the tetragonal phase and of Nb and Mn in the coexisting orthorhombic phase is observed; it agrees with the behavior of similar intergrowths elsewhere. The sixth case is noteworthy because of the higher Ta:Nb ratio in the ixiolite in comparison with the coexisting niobian rutile. This enrichment of Nb and Fe from core to rim represents an anomalous pattern of fractionation behavior.