Fluorine-, yttrium- and lanthanide-rich cerianite-(Ce) from carbonatitic rocks of the Kerimasi volcano and surrounding explosion craters, Gregory Rift, northernTanzania A. N. ZAITSEV 1,2, *, A. R. CHAKHMOURADIAN 3 , O. I. SIIDRA 4 , J. SPRATT 2 , C. T. WILLIAMS 2 , C. J. STANLEY 2 , S. V. PETROV 5 , S. N. BRITVIN 4 AND E. A. POLYAKOVA 1 1 Department of Mineralogy, Faculty of Geology, St Petersburg State University, University Emb., 7/9, St Petersburg 199034, Russia 2 Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, UK 3 Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada 4 Department of Crystallography, Faculty of Geology, St Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg 199034, Russia 5 Department of Mineral Deposits, Faculty of Geology, St Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg 199034, Russia [Received 6 April 2011; Accepted 23 August 2011] ABSTRACT Cerianite-(Ce), ideally CeO 2 , occurs as rounded grains up to 5 mm across in a block of highly altered calcite carbonatite lava from the Kerimasi volcano, and as euhedral crystals up to 200 mm across in carbonatite-derived eluvial deposits in the Kisete and Loluni explosion craters in the Gregory Rift, northern Tanzania. X-ray powder diffraction data (a = 5.434(5) A ˚ ) and Raman spectroscopy (minor vibration modes at 184 and 571 cm 1 in addition to a strong signal at 449 cm 1 ) suggest the presence of essential amounts of large cations and oxygen vacancies in the Kisete material. Microprobe analyses reveal that the mineral contains both light and heavy trivalent rare earth elements (REE) (7.915.5 wt.% LREE 2 O 3 and 4.99.7 wt.% HREE 2 O 3 ), and that it is enriched in yttrium (7.114.5 wt.% Y 2 O 3 ) and fluorine (2.23.5 wt.%). Single-crystal structure refinement of the mineral confirms a fluorite-type structure with a cationanion distance of 2.3471(6) A ˚ . The cerianite-(Ce) is considered to be a late-stage secondary mineral in the carbonatitic rocks. KEYWORDS: cerianite-(Ce), ceria, oxyfluoride, carbonatite, Raman spectroscopy, chemical composition, X-ray diffraction, crystal structure, Kerimasi, Kisete, Loluni, Tanzania. Introduction CERIANITE-(CE), ideally CeO 2 , was originally described from a xenolith in a carbonatite dyke cross-cutting ‘‘nephelinised hybrid gneiss’’ at Lackner Lake, Ontario, Canada (Graham, 1955). It has been identified as a secondary mineral at many localities worldwide, including weathered phonolites and nepheline syenites at Morro do Ferro, Brazil (Frondel and Marvin, 1959), hydrothermal veins of probable carbonatitic origin at Karonge, Burundi (Van Wambeke, 1977), granite pegmatite at Neso ¨ya, East Antarctica (Matsumoto and Sakamoto, 1982), alluvial deposits on the Afu Hills pegmatites, Nigeria (Styles and Young, 1983), weathered carbonatites at Mount Weld, Australia (Lottermoser, 1987), weathered syenite at Akongo, Cameroon (Braun et al., 1990), palaeo- sols at Flin Flon, Canada (Pan and Stauffer, 2000), skarns at Bastna ¨s, Sweden (Holtstam and * E-mail: burbankite@gmail.com DOI: 10.1180/minmag.2011.075.6.2813 Mineralogical Magazine, December 2011, Vol. 75(6), pp. 2813–2822 # 2011 The Mineralogical Society