Original paper
Preservation of the samarskite structure in a metamict ABO4 mineral: a key to crystal structure identification
Tomašić, Nenad; Gajović, Andreja; Bermanec, Vladimir; Linarić, Maša; Rajić Su, Dangsheng Š koda
European Journal of Mineralogy Volume 22 Number 3 (2010), p. 435 - 442
published: Jun 23, 2010
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)Σ1.034(Nb0.707Ta0.267Ti0.069W0.017Sn0.007)Σ1.067O4. 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.
Keywords
samarskite • low-temperature modification • metamictisation • pre-metamict structure • tem