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

Autori
Tomašić, Nenad ; Gajović, Andreja ; Bermanec, Vladimir ; Rajić Linarić, Maša ; Su, Dangsheng ; Škoda, Radek

Izvornik
European journal of mineralogy (0935-1221) 22 (2010), 3; 435-442

Vrsta, podvrsta i kategorija rada
Radovi u časopisima, članak, znanstveni

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

Sažetak
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.

Izvorni jezik
Engleski

Znanstvena područja
Fizika, Geologija, Kemija

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