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We here present reults of a crystal-chemical study of a suite of pyrochlore-group minerals from the holocrystalline syenitic ejecta scattered within the Plio-Pleistocene pyroclastic deposits of Latium, in central Italy. Despite their high actinide content, the samples studied are non-metamict owing to their young age of formation, as suggested by the well-defi ned and sharp peaks in the X-ray powder-diffraction patterns. The measured a unit-cell dimension is correlated with the (Ti + Fe 3+) content at the B site of the structure. Following the IMA classifi cation scheme for pyrochlore-group minerals modifi ed according to the "50% rule" adopted in 1992, the samples are classifi ed as betafi te, pyrochlore and uranpyrochlore. In comparison with known data for pyrochlore-group minerals, the samples have extremely high Ti contents (up to 81.3 atom % Ti relative to the Ti + Nb + Ta sum) and signifi cant and variable contents of Zr (up to 8.44 wt.% ZrO 2) and W (up to 12.14 wt.% WO 3). Some crystals display strong compositional zoning, consistent with the substitution 3Nb 5+ + Na + 3Ti 4+ + U 4+. The accommodation of extremely high amounts of W 6+ can be related to the coupled substitution 2(Nb + Ta) 5+ Ti 4+ + W 6+ at the B site. Pyrochlore-group minerals from Latium have signifi cantly high REE contents (up to 7.75 wt.% oxide). Chondrite-normalized patterns follow the general trend typical of unaltered pyrochlore, i.e., with an extreme Ce-enrichment and a decrease toward the heavy REE.

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