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Preservation of the samarskite structure in a metamict ABO4 mineral: a key to crystal structure identification

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 general formula (REE0.322Ca0.269Fe0.231U0.142Mn0.031Pb0.011Th0.010Zr0.010Al0.004Sc0.002 Na0.001K0.001)1.034(Nb0.707Ta0.267Ti0.069W0.017Sn0.007)1.067O4. The structure identified by transmission electron microscopy (TEM) techniques is well related to the proposed low-temperature samarskite modification (Sugitani et al. 1985) crystallising orthorhombically with the cell parameters a=5.68(1) Å, b=4.94(1) Å, c=5.24(2) Å. The observed diffraction maxima conform well to Pbcn space group. The original mineral sample showed to be X-ray amorphous. Gradual recrystallisation in air in the temperature range between 400 and 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 evidenced by occurrence of altered domains in the sample.

samarskite ; low-temperature modification ; metamictisation ; pre-metamict structure ; TEM

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