The Crystal Chemistry of Non-Metamict Pyrochlore-Group Minerals From Latium, Italy

Abstract

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.

Enthalpies of formation of rare earth niobates, RE3NbO7

“…This can explain why in nature pyrochlores containing Nb and Zr are relatively rare in comparison to those containing Nb and Ti. This also suggests that the traces of ZrO 2 found in ores containing pyrochlore are indeed the effect of formation of separate phases, rather than an effect of exchange reactions between niobates and zirconates (Caprilli et al 2006;Yaroshevskii and Bagdasarov 2008). The calorimetric data obtained here refer to the stability at room temperature of the indicated structures produced by cooling samples from high temperature.…”

Liandratite from Karkonosze pegmatites, Sudetes, Southwestern Poland

“…It is based on the observation that the primary mineral is altered first to a phase with the composition of the pyrochlore group minerals Table S2. Please refer also to Hogarth and Horne (1989), Lumpkin and Ewing (1996), Uher et al (1998), Chakhmouradian and Mitchell (2002), Zurevinski and Mitchell (2004), Caprilli et al (2006), Monchoux et al (2006), Mokhov et al (2008), Abd El-Naby (2009), Timofeev and Williams-Jones (2015) or directly to Pb-, Bi-, U-rich pyrochlores, and then possibly to liandratite (i.e. Fig.…”

Synthesis and characterisation of the uranium pyrochlore betafite [(Ca,U)2(Ti,Nb,Ta)2O7]

“…The degree of metamictisation increases with age of formation [7][8][9]. The mineral is also susceptible to alteration with loss of ions from the A site (mainly Ca 2+ ) and the incorporation of ions such as K, Sr, Ba, Pb or H 2 O into the structure [10][11][12].…”