ORIGINAL PAPER N. Tomasˇ ic´ Æ A. Gajovic´ Æ V. Bermanec D. S. Su Æ M. Rajic´ Linaric´ Æ T. Ntaflos R. Schlo¨gl Recrystallization mechanisms of fergusonite from metamict mineral precursors Received: 16 September 2005 / Accepted: 9 December 2005 / Published online: 14 February 2006 Ó Springer-Verlag 2006 Abstract The metamict state and recrystallization of fergusonite in metamict natural samples were studied by thermal methods (TGA-DTA), X-ray powder diffrac- tion (XRD), Raman spectroscopy (RS), transmission electron microscopy (TEM), selected area electron dif- fraction (SAED), and electron microprobe (EPMA). Two metamict mineral samples of fergusonite were investigated in order to identify the original premetamict crystal structure and to identify recrystallization mech- anisms. The TEM data and RS provided evidence on the partial preservation of the original structure in the investigated minerals, which are X-ray amorphous. It was shown that fergusonite could recrystallize from a metamict mineral with original fergusonite structure or from metamictized pyrochlore, which was altered before or after metamictization. Two recrystallization mecha- nisms were recognized: (a) epitaxial growth occurring at the boundary between preserved premetamict structure fragments and completely metamictized areas, and (b) nucleation-crystal growth mechanism occurring in completely amorphous areas of the minerals, and resulting in recrystallization of the original mineral as well as in the crystallization of a new mineral with a modified chemical composition as compared to the ini- tial matrix. Keywords Fergusonite Æ Metamict state Æ Recrystallization mechanisms Æ Raman spectroscopy Æ TEM Introduction Fergusonite is a mineral frequently found in the metamict state, and chemically described with a general chemical formula ABO 4 (A = REE, Ca, U and Th; B = Nb, Ta and Ti). Fergusonite-like phases encom- pass a large number of synthetic compounds which are of interest due to their dielectric properties. Two structures have been reported for fergusonite mineral samples. Komkov (1959) determined the schee- lite-type structure for the tetragonal fergusonite from Urals with the space group I4 1 /a, later named a-fergus- onite (Gorshevskaya et al. 1961) or T phase for synthetic YNbO 4 analogues (Yashima et al. 1997). On heating at 1,000°C, the mineral transforms to a monoclinic phase with the I2 space group and distorted scheelite structure, named b-fergusonite (Gorshevskaya et al. 1961) or M phase in synthetic fergusonite (Markiv et al. 2002). Komkov (1959) suggested tetrahedral coordination for B-cations in both a- and b-fergusonite, while Weitzel and Schro¨cke (1980) refined the structure of synthetic b-fergusonite (YNbO 4 ), and proposed the space group C2/c with 4+2 coordination for B-cations. Blasse (1973) investigated vibrational spectra of both YNbO 4 and YTaO 4 having b-fergusonite structure, and observed fewer bands than expected by symmetry considerations. This was explained by a slight distortion of b-fergusonite structure from sheelite structure that has higher sym- metry. Additionally, another monoclinic fergusonite phase named M¢ phase was determined for synthetic YTaO 4 (Wolten and Chase 1967) with the P2/a space group. This phase is obtained for the crystals grown below the M–T transformation temperature (Wolten N. Tomasˇic´ (&) Æ V. Bermanec Faculty of Science, Institute of Mineralogy and Petrography, University of Zagreb, Horvatovac bb, 10000, Zagreb, Croatia E-mail: ntomasic@jagor.srce.hr Tel.: +385-1-4605909 Fax: +385-1-4605998 A. Gajovic´ Rudjer Bosˇkovic´ Institute, POB 180, 1002, Zagreb, Croatia D. S. Su Æ R. Schlo¨gl Fritz Haber Institut der Max-Planck-Geselschaft, 14159, Berlin, Germany M. Rajic´ Linaric´ Brodarski Institut, 10000, Zagreb, Croatia T. Ntaflos Department of Geological Sciences, University of Vienna, Althastr. 14, 1090, Vienna, Austria Phys Chem Minerals (2006) 33: 145–159 DOI 10.1007/s00269-006-0061-6