EUΓ . J. Mineral. 1989, 1, 723-725 SHORT NOTE First occurrence of brithoIite-(Ce) in Italy (Monte So in m a, Vesuvius) PAOLO ORLANDI*, NATALE PERCHIAZZI** and GREGORIO MANNUCCI*** * Dipartimento di Scienze della Terra dell'Università di Pisa, Via S.Maria 53 , 56100 Pisa, Italy ** Museo di Storia Naturale dell'Università di Pisa, Certosa di Calci, 56011 Calci, Pisa, Italy *** Dipartimento di Scienze della Terra dell'Università di Milano, Via Botticelli 23 , 20133 Milano, Italy Abstract : The first occurrence of britholite-(Ce) in Italy consists of small (0.2 mm) transparent pale blue hexagonal tabular crystals in volcanic ejecta, mainly composed of sanidine, from Monte Somma, Vesuvius. The mineral is hexagonal, space group P6 3 /m (or P6 3 ) a = 9.597(9), c = 7.04(1) Å. The anhydrous crystal-chemical formula, derived from electron microprobe analysis, is : Ca 4 (REE 3 xCax 54 Th! 2 3U 0 .o8)5.95(Si5.42Po.94)6.36O26 with REE = Ce! 77, La 070 , Nd 047 , Pr 0 16 . Britholite-(Ce) from Monte Somma has a high calcium content and the highest thorium content so far reported for this mineral. It is uniaxial negative, with n ω = 1.772(2). Key-words : britholite-(Ce), Vesuvius, chemistry, X-ray data. Introduction An extraordinary variety of substitutions are known, within the general formula of the apa- tite group : (A 1 ) 4 (A 2 )6(X04) 6 Z2. The fact that apatite and britholite are isostructural was first pointed out by Machatschky (1939) and then confirmed by various other authors (Gay, 1957 ; Kupriyanova & Sidorenko, 1963). An ideal britholite-(Ce), CaΓ Ce^ + (Si0 4 ) 6 (OH) 2 , can be simply obtained from apatite by coupled substitutions of Ce 3+ for Ca 2+ in the seven- coordinated A 2 site and Si 4+ for P 5+ in the X tetrahedral site. In britholites, although the nine-coordinated A λ site is nearly always fully occupied by calcium, a great number of cations such as Ca 2+ , Al 3+ , Y 3+ , REE 3+ , Th 4+ , can be hosted in the A 2 site, whereas the tetrahedral X site is filled with Si 4+ and P 5+ . The large chemical variability of britholites leads to a series comprising many varieties with two known natural end-members, britholite-(Ce) and britholite-(Y), (Levinson, 1966). Discredited names within the britholite series are : — beckelite (Morozewich, 1904) = britholite- (Ce)(Kudrina eta /., 1961), — lessingite (Zilbermintz, 1929) = britholite- (Ce)(Kudrina eta /., 1961), — pravdite (NurΓ baev, 1962) = britholite-(Ce) (Tarkhanova eta /., 1964), — fenghuangite (Peng & Liu, 1962) = thorian britholite-(Ce) (Fleischer, 1963), — alumobritholite (Kudrina et al., 1961) = aluminian britholite-(Ce) (I.M.A. Commission on New Minerals and Mineral Names, 1966), — abukumalite (Hata, 1938) = britholite-(Y) (Levinson, 1966). Vesuvian britholite-(Ce) was found, associa- ted with biotite, in a sanidinitic block at San Vito quarry, Monte Somma. The occurrence consists of only three very small (0.2 mm) transparent pale blue tabular hexagonal crys- tals, lying on sanidine crystals. The sample and the crystals studied are preserved in the Museo di Storia Naturale dell'Università di Pisa. 0935-1221/89/0001-723 $ 0.75 © 1989 E. Schweizerbart'sche Verlagsbuchhandlung, D-7000 Stuttgart 1