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Recovery and decomposition of euxenite structure in a Nb-Ta-REE oxide mineral from Ampangabe, Madagascar

Euxenite crystallizes in space group Pbcn with general formula AB2O6. A-site is occupied by REE, Ca, Th, U, and Fe, while Nb, Ta and Ti are positioned in B-site. Due to  -decay of Th and U, euxenite is frequently found to be partly or completely metamictized (amorphized). Structure recovery is normally accomplished in heating experiments. The investigated sample was originally from Ampangabe, Madagascar, showing no crystal faces and having vitreous luster, conchoidal fracture and brownish red color. Preliminary XRD measurements indicated that the mineral was heavily metamictized, showing only three very weak and broad diffraction maxima of euxenite structure. The mineral was gradually recrystallized at 400, 500, 650, 800 and 1000°C in normal atmosphere for 24 hours at each temperature. The recrystallization of euxenite structure was observed continuously up to 800°C. At 800°C petscheckite was found to crystallize along with euxenite recrystallization. However, at 1000°C the euxenite structure disintegrated into a multiphase system containing pyrochlore, petscheckite and high-temperature samarskite phase. TEM micrographs of thermally untreated euxenite showed large metamict domains, some of them containing isolated crystalline relics of premetamict structure. Both SAED and HRTEM micrographs confirmed these crystalline domains have euxenite structure. SAED data yielded the following unit cell parameters for euxenite: a=14.63(4) Å , b=5.50(2) Å , c=5.19(2) Å , and V=418 (1) Å 3. SAED data of the mineral portion heated at 1000°C confirmed the multiphase system, already indicated by XRD data. HRTEM micrographs showed clear phase boundaries sometimes appearing as stacking faults. Although euxenite is considered to be a high-temperature phase in REE-Nb-Ta-Ti systems of AB2O6 stoichiometry [A. I. Komkov, Dokl. Akad. Nauk 206 (1972), 1353], and its recrystallization is usually straightforward, this decomposition of euxenite structure could be assigned to a high Fe content, as indicated by EPMA analyses. When calculated on 6-oxygen basis, there is 0.532 of Fe2+ per euxenite formula. This is a relatively high amount of Fe2+ compared to Y content (0.434), which is normally dominant A-site cation. Consequently, Fe2+ is too small cation to support 8-fold coordination for A-site cations in euxenite structure. Thus, at temperatures higher than 800°C, euxenite decomposes into Fe-rich phases: petscheckite and samarskite high-temperature phase, both containing A-site cations in 6-fold coordination [M. Gasperin, Bull. Soc. franc. Miner. Crist. 83 (1960) 1 ; J. K. Warner & R. C. Ewing, Am. Mineral. 78 (1993), 419]. On the other hand, Y is accommodated in a pyrochlore phase with 8-fold coordinated A-site cations.

euxenite; metamictization; recrystallization; Ampangabe; Madagascar

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