Abstract
AbstractThe crystal chemistry and crystal structure of the rare earth element phosphates, monazite-(Ce), Ce(PO4), and xenotime-(Y), Y(PO4), as well as the arsenates, gasparite-(Ce), Ce(AsO4), and chernovite-(Y), Y(AsO4), from the hydrothermal quartz-bearing fissures, related to pegmatites overprinted by amphibolite facies, cropping out at Mt. Cervandone, Western Alps, Piedmont, Italy, have been investigated by means of electron microprobe analysis in wavelength dispersion mode and single-crystal X-ray diffraction. The chemical data reveal the occurrence of a full solid solution among the isostructural chernovite-(Y) and xenotime-(Y) with tetragonal symmetry, whereas a wide miscibility gap is observed for the isostructural gasparite-(Ce) and monazite-(Ce) of Mt. Cervandone, with monoclinic symmetry. A significant chemical heterogeneity has been observed for several investigated samples, especially related to the Th content, which is locally enriched in ThSiO4 grains. The analysis of the refined structural models demonstrates the significant control played by the composition of the tetrahedrally-coordinated (As,P)-bearing sites on the bulk unit-cell volume, and on the size and shape of the (REE)-coordination polyhedra.
Highlights
According to the IUPAC definition (Connelly et al 2005), the Rare Earth Elements (REE) consist in a group of 17 elements, comprising the lanthanides (La-Lu or Ln series), Y and Sc, characterized by a similar geochemical behavior
The composition of gasparite-(Ce) and monazite-(Ce) crystals is closer to the ideal endmembers, and only a partial solution is observed
An almost complete solid solution has been found along the chernovite-(Y) and xenotime-(Y) join, whereas a wide miscibility gap occurs among the investigated end-members of gasparite-(Ce) and monazite-(Ce) series from Mt
Summary
According to the IUPAC definition (Connelly et al 2005), the Rare Earth Elements (REE) consist in a group of 17 elements, comprising the lanthanides (La-Lu or Ln series), Y and Sc, characterized by a similar geochemical behavior. REE are used in several applications, including permanent magnets, phosphors, catalytic converters, or as glass and metal alloys additives Their mining exploitation is largely concentrated in China (U.S Geological Survey, 2021) and their commodities export policy changed significantly in the last decade, introducing export quotas and taxes (Mancheri, 2015). The crystal structure of these minerals has been object of a large number of studies and reviews (e.g., Mooney, 1948; Boatner, 2002; Ni et al 1995; Kolitsch and Holtstam, 2004b; Clavier et al 2011) These ATO4 compounds show two possible structural arrangements (Fig. 1): a monoclinic monazite-type structure (Fig. 1a) and a tetragonal zircon-type ( known as “xenotime-type”) structure (Fig. 1b). Mills et al (2010), along with arsenoflorencite(La), reported the presence of chernovite-(Y) in Mn-rich nodules from the Grubependity Lake cirque, Komi Republic, Urals
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