REE-bearing petscheckite from Tiltvika, Nordland, Norway, and its heating products

Abstract

X-ray powder-diffraction investigation of a mineral sample from the Tiltvika granite pegmatite, Norway, shows the presence of a metamict phase with ferrocolumbite inclusions. On heating at 650 °C for 21 hours, the diffraction pattern of petscheckite, a hexagonal mineral with idealized formula U 4 + Fe 2 + (Nb,Ta) 2 O 8 , is observed. The Tiltvika petscheckite is rich in REEs and has the general formula (U,REE)(Fe 2 + , Fe 3 + )(Nb,Ta) 2 (O,OH) 8 . Its unit-cell parameters are a = 6.42(2), c = 4.02(1) A. Heating at 1000 °C for 21 hours yields a phase with columbite-type structure and a pattern very close to that reported for heated wolframoixiolite. Assuming space group Pcan, this phase has unit-cell parameters a = 5.005(5), b = 14.05(2), c = 5.64(1) A. The high content of α-emitting uranium could explain the metamictization of a primary petscheckite. On the other hand, it is possible that an unknown primary mineral with complex composition has disintegrated into microexolutions of phases with simple stoichiometry, which have recombined to petscheckite on heating at 650 °C. For practical purposes, however, petscheckite as a metamict mineral species must be defined by its chemical composition and the characteristic X-ray powder pattern that it gives on heating to 650 °C.