Naslov
Recrystallization of metamict Nb-Ta-Ti-REE complex oxides:an X-ray diffraction and Raman spectroscopy study

Autori
Tomašić, Nenad ; Gajović, Andreja ; Bermanec, Vladimir

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

Izvornik
Twelfth Croatian-Slovenian Crystallographic Meeting : Book of Abstracts / Cetina, Mario ; Kajfež, Tanja ; Popović, Stanko ; Štefanić, Zoran - Zagreb : Hrvatska akademija znanosti i umjetnosti (HAZU) ; Hrvatska Kristalografska Zajednica, 2003, 29-29

Skup
Twelfth Croatian-Slovenian Crystallographic Meeting

Mjesto i datum
NP Plitvička jezera, Hrvatska, 19.06.2003. - 22.06.2003

Vrsta sudjelovanja
Predavanje

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
Nb-Ta-Ti-REE complex oxides; metamictization; recrystallization; X-ray diffraction; Raman spectroscopy

Sažetak
Recrystallization of metamict Nb-Ta-Ti-REE complex oxides: an X-ray diffraction and Raman spectroscopy study N. Tomašić1, A. Gajović2, V. Bermanec1 1University of Zagreb, Faculty of Science, Institute of Mineralogy and Petrography, Horvatovac bb, HR-10000 Zagreb, Croatia ; E-mail: ntomasic@jagor.srce.hr 2 Ruđer Bošković Institute, POB 180, 10002 Zagreb, Croatia The complex oxides of Nb, Ta, Ti and REE originating from granitic pegmatites are frequently characterized by metamict state of their crystal structure. The term metamict is related to those mineral phases found amorphous in nature but considered to be originally crystalline [J. Berman, Am. Mineralogist 40 (1955) 805]. Here, the process of recrystallization of metamict Nb-Ta-Ti-REE complex oxides induced by the annealing experiments was monitored using X-ray powder diffraction and Raman spectroscopy. The samples of metamict aeschynite-(Y) [(Y, Ca, Fe, Th)(Ti, Nb)2O6], polycrase-(Y) [(Y, Ca, Ce, U, Th)(Ti, Nb, Ta)2O6] and fergusonite-(Y) [YNbO4] were used for the investigation. The mineral samples were heated in air in muffle furnace at 400, 500, 650, 800 and 1000°C for 24 hours in each case. The powder diffraction patterns for all the unheated minerals evidence their metamict state. Expectedly, as the minerals are heated gradual recrystallization takes place. With temperature increase the diffraction peaks become narrower and more intense. The mineral samples heated at 1000°C show the highest degree of crystallinity and their corresponding unit cell parameters are as follows: Aeschynite-(Y) Pbnm a=5.193(1) &Aring ; , b=10.932(3) &Aring ; , c=7.383(2) &Aring ; , V=419.1(1) &Aring ; 3 Polycrase-(Y) Pcan a=5.544(2) &Aring ; , b=14.517(3) &Aring ; , c=5.1806(9) &Aring ; , V=416.9(1) &Aring ; 3 Fergusonite-(Y) I2 a=5.297(4) &Aring ; , b=10.964(8) &Aring ; , c=5.063(4) &Aring ; , b=94.18°, V=293.3(3) &Aring ; 3 Fergusonite-(Y) seems to undergo a phase transformation in the temperature range from 800°C to 1000°C. The fergusonite phase that could be indexed as a tetragonal phase (a=5.169(3)&Aring ; , c=5.471(6)&Aring ; , V=146.1(2)&Aring ; 3) starts to recrystalize at 400°C. Its space group is not determined [J. Berman, Am. Mineralogist 40 (1955) 805] but future experiments could help in its revealing. At 1000°C monoclinic fergusonite phase seems to predominate, although stronger diffraction peaks of the tetragonal phase are still present. Aeschynite-(Y) and polycrase-(Y) show continuous recrystallization from 400°C to 1000°C into the ordered orthorhombic phases. Preliminary results of the Raman spectroscopy investigation generally confirm a gradual recrystallization of the investigated phases as the temperature rises. The general trend of Raman bands sharpening and intensifying with the temperature increase is observed suggesting the structure ordering. This observation is in accordance with the differences in Raman spectra for the zircon samples with various degree of metamictization [L. Nasdala, G. Irmer, D. Wolf, Eur. J. Mineral. 7 (1995) 471-478]. The fergusonite tetragonal phase gives more differentiated Raman bands in the wavenumber range 150-500 cm-1 if compared to the monoclinic phase crystallizing at the higher temperature. Also, a few bands are observed for unheated fergusonite suggesting some local structure ordering not observable by X-ray diffraction. After heating fergusonite at 400°C these bands become less pronounced, but appear more intensively at higher temperatures. The Raman spectra of aeschynite-(Y) and polycrase-(Y) show a similar appearance as the minerals are heated emphasizing their compositional and structural similarity. Still, differences in position, shape and intensity of the bands could be observed. A comparison of the Raman spectra for the investigated minerals is only possible with synthetic compounds of the similar chemistry since the vibrational spectra data for metamict minerals are sparse in literature.

Izvorni jezik
Engleski

Znanstvena područja
Fizika, Geologija

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