Introduction Rare Zr-Ti-Nb disilicates are typical accessory minerals of nepheline syenites and carbonatites. During petro- graphic studies, their identification is usually based on electron-microprobe chemical data, as, for example, by Woolley & Platt (1986), Mariano & Roeder (1989), Cundari & Ferguson (1991), Keller et al. (1995). A scrutiny of these chemical analyses shows that all of them can be recalculated on the basis of the general formula M 16 (Si 2 O 7 ) 4 (O,OH,F) 8 ; M represents cations with coordi- nation ranging from VI to VIII, from the small-radius and high-charge Ti 4+ , Zr 4+ , Nb 5+ to REE 3+ , Mn 2+ , Fe 2+ and, last, to the high-radius and low-charge cations Na + , Ca 2+ . Actually, the crystal-chemical formula M 16 (Si 2 O 7 ) 4 (O,OH,F) 8 is representative of at least three distinct mineral groups, namely the cuspidine-låvenite, the rosenbuschite and the rinkite families, for a total of 18 distinct mineral species, as it is shown in Table 1. A wide range of possible substitutions can be expected, and indeed is shown by the minerals of these groups. On the other hand, minerals with quite different structural arrangement often display germane chemical composi- tions, as for rosenbuschite (Shibayeva et al., 1964), hiort- dahlite I (Merlino & Perchiazzi, 1985) and hiortdahlite II (Merlino & Perchiazzi, 1987). Therefore, due to the rela- tively high range of possible chemical substitutions in each group and to the identical crystal-chemical formula for the minerals in the three groups, an unambiguous phase iden- tification may be difficult on the basis of the chemical composition alone. Zr,Ti-disilicates were also found as accessories in the pegmatitoid facies of venanzite from the Pleistocene lava flow of Pian di Celle, and were intensively studied by various authors after the first finding by Cundari & Ferguson (1991). Cundari & Ferguson (1991, 1994), just on the basis of microprobe data, assigned them to the mineral species götzenite. A more detailed study of Zr,Ti-disilicates is due to Sharygin et al. (1996), who reported comprehen- sive data of chemistry, spectroscopy and powder X-ray diffractometry (XRD). On this basis, they identified götzenite, Zr-rich cuspidine and Zr,Ti-rich cuspidine. About this last phase, they hypothesized it could be the result of a solid solution between cuspidine and hiort- dahlite-I. We were prompted to study these minerals by our inves- tigations about phases with general formula M 16 (Si 2 O 7 ) 4 (O,OH,F) 8 and we found that, as pointed out by Sharygin et al. (1996), powder XRD is not the ideal tech- nique to unambiguously identify these minerals. In fact, if it is possible through powder XRD data to assign the minerals to one of the three above mentioned distinct Eur. J. Mineral. 2004, 16, 957-969 Chemical and structural study of the Zr,Ti-disilicates in the venanzite from Pian di Celle, Umbria, Italy MARCO BELLEZZA*, STEFANO MERLINO and NATALE PERCHIAZZI Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, I-56126 Pisa, Italy Abstract: Cuspidine and götzenite were found as accessories in the pegmatitoid facies of venanzite from the Pleistocene lava flow of Pian di Celle. The modal compositions of venanzite and its pegmatitoid facies have been defined through combined XRF and Rietveld quantitative analyses. Cuspidine and götzenite were unambiguously identified by combining microprobe analyses and structural studies. The structure analyses were carried on with single-crystal XRD data and the refinements converged to R 1 = 0.048 and 0.053 for cuspidine and götzenite respectively. Cuspidine from Pian di Celle, ideally (Ca,Zr) Ca 2 (Ca,Na) Si 2 O 7 F 2 , Z = 4, space group P2 1 /a, a = 10.919(2), b = 10.485(1), c = 7.485(1) Å, β = 109.55(1)°, represents a Zr,Na-rich variety of cuspidine. Götzenite from Pian di Celle, ideally (Ca,Zr) (Ca,Na) Ca (Na,Ca) 0.5 Ti 0.5 Si 2 O 7 (F,OH,O) 2 , Z = 2, space group P – 1, a = 9.636(3), b =7.341(2), c =5.737(1) Å, α = 89.94(2), β = 100.74(2), γ = 101.06(3)°, represents a Zr-rich variety of götzenite. Zirconium and sodium enter in the structures substituting part of calcium, according to the main substitution indicated by the chemical data: 2Ca 2+ + F - ↔ Na + + (Zr+Ti) 4+ + O 2- . Key-words: cuspidine, götzenite, microprobe chemical data, single-crystal X-ray diffractometry, crystal structure. 0935-1221/04/0016-0957 $ 5.85 © 2004 E. Schweizerbart’sche Verlagsbuchhandlung. D-70176 Stuttgart DOI: 10.1127/0935-1221/2004/0016-0957 *E-mail: bellezza@dst.unipi.it