engleski

Incomplete recovery of euxenite structure from a heavily metamictized mineral

Euxenite is a complex oxide mineral with general chemical formula AB2O6 crystallizing in the space group Pbcn. A-site cations are predominantly REE partially replaced by Ca, U and Th, while Nb, Ta and Ti occupy the B-site in the crystal structure. The crystal structure of euxenite is frequently characterized by radiation damage due to self-irradiation which is mainly caused by the  -decay products of U and Th. Thus heavily damaged mineral samples are characterized by a certain degree of amorphous state, in this case called metamict state. A heavily metamictized mineral sample predetermined as euxenite and originating from Lauvrak, Norway, was investigated in order to determine the original structure and its recovery by annealing experiments. The annealing experiments were performed in air at 400, 500, 650, 800 and 1000°C for 24 hours in each case. Original and thermally treated mineral portions were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM, HRTEM) and selected area electron diffraction (SAED). The X-ray diffraction pattern of the unheated mineral shows a high degree of metamictization of the original structure (Fig. 1). There is only recognized a low intensity reflection, which can be assigned to the 211 reflection of the euxenite structure. With temperature increase the mineral recrystallizes showing diffraction patterns characteristic for the euxenite structure. The diffraction data indicate a slow rate of recrystallization which increases from 650°C above. However, at 1000°C the euxenite phase disintegrates into a pyrochlore phase (A2-mB2O6(O, OH)1-n) and a phase which could be related to the estimated high-temperature samarskite phase (ABO4) [1]. SAED and HRTEM images of the unheated sample (Fig. 2) show a partial preservation of premetamict structure. The d-values calculated from SAED pattern (Fig. 2a2) and HRTEM image (Fig. 2b) are characteristic for the euxenite structure. The calculation of unit cell parameters from SAED pattern for the unheated sample yields a=14.49(6) Å , b=5.63(2) Å , c=5.16(6) Å , V=421(5) Å 3, what corresponds well to the euxenite structure parameters. The presented parameters do not indicate a unit cell swelling which is frequently observed due to radiation damage of the crystal lattice. It is likely that significant iron content of A-site (0.371 apfu on 6-oxygen basis) causes the shrinkage of the euxenite cell parameters, which is here compensated by the unit cell swelling resulting in average unit cell parameters for euxenite. The high iron content could be responsible for the incomplete recovery of euxenite structure and its disintegration at higher temperatures, although euxenite structure is considered to be stable at high-temperature [2]. The euxenite structure can be related to the columbite structure, with exception of A-site cations being accommodated in 8-fold coordination in the euxenite and 6-fold coordination in columbite structure. Iron on A-site supports octahedral coordination, meanwhile REE, which are dominant A-site cations in euxenite, require 8-fold coordination. This could be a reason for the disintegration of the recrystallizing euxenite structure to a pyrochlore (8-fold coordination of A-cations) and the assumed high-temperature samarskite phase with 6-fold coordinated A-cations at 1000°C [3]. On the other hand, the relics of a pyrochlore phase in the original sample, although not observed, cannot be excluded.

euxenite; metamictization; recrystallization; XRD; TEM

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