Preservation of samarskite structure in a metamict ABO4 mineral: a key to crystal structure identification
| Authors | |
|---|---|
| Year of publication | 2010 |
| Type | Article in Periodical |
| Magazine / Source | European Journal of Mineralogy |
| MU Faculty or unit | |
| Citation | |
| Field | Geology and mineralogy |
| Keywords | samarskite; low-temperature modification; metamictisation; pre-metamict structure |
| Description | 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 (REE0.322Ca0.269Fe0.231U0.142Mn0.031Pb0.011Th0.010Zr0.010Al0.004Sc0.002Na0.001K0.001)1.034(Nb0.707Ta0.267Ti0.069W0.017Sn0.007)1.067O4 which corresponds well to the general ABO4 stoichiometry of the samarskite group. The structure identified by transmission electron microscopy (TEM) techniques is well related to the proposed low-temperature samarskite modification (Sugitani et al. 1985) crystallizing in Pbcn space group with the cell parameters a=5.69(2) A, b=4.91(2) A, c=5.21(2) A. The original mineral sample showed to be X-ray amorphous. Gradual recrystallisation in air in the temperature range between 400 and 1000oC yielded a pyrochlore phase being stable at all experimental temperatures. At 800oC a high-temperature samarskite phase (s.g. P2/c) co-crystallised with an increase in crystallinity at 1000oC. 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. Raman spectra of thermally untreated samarskite showed generally metamict nature of the mineral, although a few normal Nb-O vibration modes can be resolved indicating BO6 arrangement similar to the one of columbite. |
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