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Incorporation of REE into leucophanite: a compositional and structural study

H. Friis^1,*,, T. Bali-unic¹, C. T. Williams² and R. Garcia-Sanchez²

¹ Department of Geography and Geology, University of Copenhagen, Øster Voldgade 10, 1350 København K, Denmark
² Department of Mineralogy, Natural History Museum, Cromwell Road, London, SW7 5BD, UK

^* E-mail: geofriis{at}yahoo.com

The crystal structures of nine, and the chemical compositions of ten, natural samples of leucophanite, ideally NaCaBeSi₂O₆F, were investigated. The analysed samples display a large compositional variation with trace-element abundances >50,000 ppm, primarily due to rare earth elements (REE). From these data, we propose a substitution scheme for the incorporation of REE for Ca, with additional Na substituting for Ca and the generation of vacancies to ensure charge balance. Compositional zonation was observed in some samples; this zonation correlates with variations in cathodoluminescence. The crystal structure of the nine analysed samples could all be refined in space group P2₁2₁2₁. We found no evidence for a reduction of symmetry with increased trace-element concentration. Various twin combinations were observed and these seem related to crystallization conditions rather than structural or chemical factors.

KEYWORDS: leucophanite, REE, crystal-chemistry, alkaline complexes, crystal twins