The crystal-chemistry of non-metamict pyrochlore-group minerals from Latium (Italy)

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-defined and sharp peaks in the X-ray powder-diffraction patterns. The measured a unit-cell dimension is correlated with the (Ti + Fe3+) content at the B site of the structure. Following the IMA classification scheme for pyrochlore-group minerals modified according to the “50% rule” adopted in 1992, the samples are classified as betafite, 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 significant and variable contents of Zr (up to 8.44 wt.% ZrO2) and W (up to 12.14 wt.% WO3). Some crystals display strong compositional zoning, consistent with the substitution 3Nb5+ + Na+ ⇌ 3Ti4+ + U4+. The accommodation of extremely high amounts of W6+ can be related to the coupled substitution 2(Nb + Ta)5+ ⇌ Ti4+ + W6+ at the B site. Pyrochlore-group minerals from Latium have significantly 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.