Naslov
Relics of samarskite structure in a metamict ABO4 mineral and its high-temperature transformations

Autori
Tomašić, Nenad ; Gajović, Andreja ; Bermanec, Vladimir ; Rajić Linarić, Maša ; Su, Dangsheng ; Ntaflos, Theodoros ; Raade, Gunnar

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, ostalo

Izvornik
19th General Meeting of the International Mineralogical Association, Program & Abstracts / Yamanaka, Takamitsu - Kobe, 2006

Skup
19th General Meeting of the International Mineralogical Association

Mjesto i datum
Kobe, Japan, 23.07.2006. - 28.07.2006

Vrsta sudjelovanja
Poster

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
samarskite; metamictization; recrystallization; Beinmyr; Norway

Sažetak
A mineral predetermined as samarskite and originating from Beinmyr, Iveland, Norway, was found to be amorphous to X-rays. The chemical analysis yielded the empirical formula (REE0.381Ca0.241Fe0.207U0.142) 0.971(Nb0.764Ta0.262Ti0.065) 1.091 that fits quite well to the one of the assumed structural models for samarskite. Due to the metamict nature of the mineral, the structure recovery was induced by annealing experiments in air at 400, 500, 650, 800 and 1000°C. The sample portions were also heated in N2 and in slightly reductive Ar/H2 atmosphere at 600, 1000 and 1300°C. The annealing experiments in air showed the start of recrystallization at 650°C with crystallization of a pyrochlore phase and the occurrence of a new phase at 1000°C, which can be characterized as a proposed high-temperature samarskite phase with wolframite-type structure (s.g. P2/c, a=5.63(1) Å , b=9.93(2) Å , c=5.19(1) Å ,  =93.6(2)°). The results of the annealing experiments in N2 and Ar/H2 atmosphere show similar recrystallization sequences like the experiments in air for 600 and 1000°C. At 1300°C, the phases that crystallized at lower temperatures are still present and there is no indication of new phases or phase transitions. Different atmospheric conditions during recrystallization seem to influence the heating products since the high-temperature samarskite phase is observed only for the mineral heated in air. For N2 and Ar/H2 atmosphere beta-fergusonite seems to be the most probable high-temperature phase along with pyrochlore. The variability of recrystallization of samarskite should be attributed to complex chemical composition, stability of the original structure, annealing conditions and heavily metamictized crystal structure, thus imparting the identification of the original structure in the thermally untreated sample. TEM micrographs of the unheated samarskite reveal the preservation of the original structure fragments in a predominantly amorphous mineral matrix. These crystalline domains show lattice fringes with d-values which could be assigned to a presumed low-temperature samarskite phase. SAED patterns obtained for the unheated sample confirmed these indications of the samarskite structure, which can be considered as the original one. The patterns might be indexed on an orthorhombic cell with Pbcn space group and calculated unit cell parameters a=5.69(2) Å , b=4.91(2) Å , c=5.21(2) Å . The low-temperature samarskite phase could be envisaged as a columbite-related structure assuming octahedral coordination for B-cations and octahedral or higher coordination for A-cations.

Izvorni jezik
Engleski

Znanstvena područja
Fizika, Geologija

POVEZANOST RADA