Abstract

Relics of the original samarskite structure were identified in a metamict ABO4 mineral from Beinmyr pegmatite, Norway, using selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). The chemical composition of the sample confirmed the affiliation to the samarskite group giving the general formula (REE0.322Ca0.269Fe0.231U0.142Mn0.031Pb0.011Th0.010Zr0.010Al0.004Sc0.002Na0.001K0.001)∑1.034(Nb0.707Ta0.267Ti0.069W0.017Sn0.007)∑1.067O4. The structure identified by transmission electron microscopy (TEM) techniques is well related to the orthorhombic low-temperature samarskite modification with cell parameters a = 5.68(1), b = 4.94(1), c = 5.24(2) Å. The observed diffraction maxima conform well to Pbcn space group. The untreated sample is X-ray amorphous. Gradual recrystallisation in air from 400 to 1000 °C yielded a pyrochlore phase being stable at all experimental temperatures. At 800 °C a high-temperature samarskite phase (s.g. P2/c) co-crystallised, with an increase in crystallinity at 1000 °C. Recrystallisation in a reducing atmosphere (Ar/H2) did not yield the high-temperature samarskite phase solely. Unsuccessful recrystallisation of the high-temperature samarskite modification is mainly related to an alteration which shifted the composition of samarskite to pyrochlore, as shown by the occurrence of altered domains in the sample.

You do not currently have access to this article.