Nb and Ta oxide phases in the Fonte del Prete granitic pegmatite Island of Elba, Italy.

The Fonte del Prete rare-element-enriched granitic pegmatite dike, located near the village of San Piero in Campo, Island of Elba, Italy, displays LCT-type geochemical features, and is genetically related to the Monte Capanne pluton. The dike is characterized by complex asymmetrical zoning, with miarolitic pockets containing primitive or highly evolved assemblages. There is a noteworthy presence of Nb and Ta oxides and complex oxides, occurring mainly in the pockets. Euxenite-(Y), polycrase- (Y), U–Bi-rich polycrase-(Y), and ferrocolumbite occur in the primitive pockets in the central part (core zone), whereas U–Birich polycrase-(Y), uranopolycrase, manganocolumbite, manganotantalite, “wolframo-ixiolite”, titanowodginite and microlite are found in the evolved pockets, at the extremities of the dike. Niobian rutile and euxenite-(Y) are also found in the massive pegmatite. The crystal chemistry and properties of the Nb and Ta oxides, as revealed by X-ray-diffraction studies, permit a reconstruction of the last stages of crystallization within each environment. During the evolution of the pegmatite, a volatile-rich melt with a high content of rare elements reached saturation in aqueous vapor. Minerals of Ti, Nb, Fe, Y + HREE occur in the primitive pockets; the enrichment of Mn, Ta and W in the evolved ones led to the crystallization of minerals with high Ta/(Ta + Nb) and Mn/(Mn + Fe) values, such as manganotantalite, microlite and titanowodginite. The polycrase-(Y) is unusually enriched in Bi and U, and is zoned from a Bi-poor core to a Bi-rich rim.