Orthovanadate wakefieldite-(Ce) in symplectites replacing vanadium-bearing omphacite in the ultra-oxidized manganese deposit of Praborna (Aosta Valley, Western Italian Alps)

Abstract

Abstract Because of their unique structure and properties, rare-earth (REE) orthovanadates have been extensively employed for decades in advanced ceramics, in particular in the laser industry in replacement of Nd:YAG. A Ca-bearing REE orthovanadate with the empirical formula (Ce0.279Ca0.271Y0.267Gd0.057Nd0.055Dy0.032Sm0.027La0.020Th0.027Sr0.002)(V0.9085+Cr0.0673+Fe0.0173+As0.0055+)O4⋅nH2O has been found in metacherts from Praborna (Italian Alps) as micrometer-sized euhedral crystals in clinopyroxene + plagioclase symplectites replacing eclogite-facies vanadium-bearing omphacite (Aegirine55–48Jadeite42–33Diopside10–8 with V2O3 ≤ 1.39 wt%). We applied the synchrotron radiation, single crystal, micro-diffraction technique, recently optimized at ID09A beamline (ESRF, France), to determine the crystal structure of this mineral. It is tetragonal and isostructural with zircon, with a = 7.2233(12) Å, c = 6.3949(18) Å, V = 333.66(13) Å3, Z = 4, space group I41/amd, and it has been therefore identified as Ca- and Y-bearing wakefieldite-(Ce) (ideally CeV5+O4). Cell parameters are in agreement with those of synthetic Ce0.7Ca0.3VO4. Raman spectra of the studied wakefieldite-(Ce) are comparable with natural and synthetic wakefieldite-(Ce) spectra and revealed the presence of OH groups and/or water of hydration, which is also suggested by the low totals in microprobe analyses. Mass balance indicates that wakefieldite-(Ce) is a by-product of the omphacite breakdown; omphacite and Mn-rich epidote, a minor reactant, provided vanadium and REE, respectively. Petrological observations and thermodynamic modeling suggest that the mineral, coexisting with hematite, Mn-rich epidote, and braunite, formed during retrogression to greenschistfacies conditions at ultra-oxidized conditions (DFMQ ≥ +16 log units), which are often observed in Mn-oxide ores. Wakefieldite is an effective scavenger of REE in oxidized geological environments at P-T conditions that range from sedimentary to medium-grade metamorphic settings, even where the REE bulk concentration is negligible. Its rarity reflects both the overall low abundance of vanadium and the need for ultra-oxidized conditions that are rarely achieved in metamorphic rocks, where REE phosphates (i.e., monazite, xenotime) are commonly found instead.