Crystal Chemistry and Cation Ordering in Zirconolite 2M
- PDF / 137,591 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 55 Downloads / 156 Views
Crystal Chemistry and Cation Ordering in Zirconolite 2M Lumpkin G.R.1,2, Whittle K.R.1, Howard C.J.2, Zhang Z.2, Berry F.J.3, Oates G.3, Williams C.T.4, and Zaitsev A.N.4 1 Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK 2 ANSTO Materials, Private Mail Bag 1, Menai 2234, NSW, Australia 3 Department of Chemistry, The Open University, Milton Keynes MK7 6AA, UK 4 Department of Mineralogy, The Natural History Museum, London, SW7 5BD, UK
ABSTRACT Structural studies of single phase or nearly single phase zirconolite ceramic samples have been conducted using electron microscopy and microanalysis, X-ray diffraction, neutron diffraction, and spectroscopic methods. We show that it is possible to produce a complete series of zirconolite 2M samples with substitution of 2Ti by Nb+Fe in the HTB layer. The samples are single phase up to about 80% Nb +Fe substitution, with the appearance of a minor perovskite phase at higher Nb+Fe levels. Electron probe microanalysis reveals that the samples are homogeneous and close to their nominal compositions, except for those containing perovskite, which have a slight excess of Zr and a deficiency in the Fe content. The lattice parameters and the positions of certain Raman bands are non-linear as a function of composition, suggesting the possibility of cation ordering over the three available Ti sites within the HTB layer. Rietveld refinement of Synchrotron X-ray powder data for the Nb+Fe end-member have been conducted for the disordered case and for six trial models each with a different ordering scheme. Results of this exercise indicate that Fe preferentially occupies the Ti2 (split) site with partial ordering of Nb and the remaining Fe over the Ti1 and Ti2 octahedra. The preference of Fe for the five coordinated Ti2 site has been confirmed by 57Fe Mossbauer spectroscopy. INTRODUCTION Zirconolite, nominally CaZrTi2O7, is a rare accessory mineral found in a wide range of rock types and geological environments. The chemical composition of natural zirconolite can vary extensively, with the main substitutions involving lanthanides (Ln), actinides (Act), Nb, and Fe. In natural systems, a number of important coupled substitutions have been documented by Gieré et al. [1]. Most of these involve the incorporation of lanthanides and actinides on the Ca site, with charge balance provided by the substitution of Mg, Al, and Fe for Ti. Zirconolite is also one of the most important actinide host phases in ceramic materials designed for the safe encapsulation and long term storage of nuclear wastes in geological repositories. Many of the substitutions observed in natural samples have been confirmed via laboratory synthesis, procedures [2-5]. One important difference between zirconolites in natural and synthetic systems involves the Zr site. This site exhibits only limited subtitution by heavy rare earths and actinides in nature, whereas the coupled substitution 2Ln → CaZr and the simple substitution U → Zr are both readily obtained by laboratory syntheses. However, the former s
Data Loading...
