Nb-Ta-Ti oxides in granitic pegmatites from the Topsham pegmatite district, southern Maine

Numerous bodies of granitic pegmatite intrude gneissic rocks in the Topsham pegmatite district, in southern Maine. Many of these pegmatites contain metamict, REE-bearing, complex Nb-Ta-Ti oxides. Initial reports identified this mineral as samarskite-(Y), an ABO ₄ mineral with Nb, Ta and Ti at the B site and REE+Y dominant at the A site. However, electron-microprobe and X-ray-diffraction analyses of seven recently collected samples from various locations in the district reveal that the oxide mineralogy is complex, consisting of several oxide phases. When heated, five of the specimens examined recrystallized to high-T samarskite plus other phases. However, these specimens have U+Th greater than REE+Y at the A site, and thus are ishikawaite rather than samarskite-(Y). Ishikawaite was found in all of the locations studied except the Consolidated Number 2 Quarry. The only metamict-appearing sample collected from the Consolidated Number 2 Quarry is ferrocolumbite, an AM ₂ O ₆ oxide. At the Stand Pipe Hill location, polycrase-(Y), an AB ₂ O ₆ oxide with Y dominant at the A site and Ti dominant at the B site, occurs in addition to ishikawaite. Samarskite-(Y) is notably absent from the pegmatites studied.