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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 (Ce 0.279 Ca 0.271 Y 0.267 Gd 0.057  Nd 0.055Dy    0.032Sm    0.027La    0.020Th    0.027Sr    0.002)    (    V   0.908    5  +Cr    0.067    3  +Fe    0.017    3  +As    0.005    5  +)    O  4   ⋅    nH   2   O    $\left. \text{D}{{\text{y}}_{0.032}}\text{S}{{\text{m}}_{0.027}}\text{L}{{\text{a}}_{0.020}}\text{T}{{\text{h}}_{0.027}}\text{S}{{\text{r}}_{0.002}} \right)\left( \text{V}_{0.908}^{5+}\text{Cr}_{0.067}^{3+}\text{Fe}_{0.017}^{3+}\text{As}_{0.005}^{5+} \right){{\text{O}}_{4}}\cdot \text{n}{{\text{H}}_{2}}\text{O}$   has been found in metacherts from Praborna (Italian Alps) as micrometer-sized euhedral crystals in clinopyroxene + plagioclase symplectites replacing eclogite-facies vanadium-bearing omphacite (Aegirine 55–48 Jadeite 42–33 Diopside 10–8  with V 2 O 3  ≤ 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) Å 3 ,  Z  = 4, space group  I 4 1 / amd , and it has been therefore identified as Ca- and Y-bearing wakefieldite-(Ce) (ideally CeV 5+ O 4 ). Cell parameters are in agreement with those of synthetic Ce 0.7 Ca 0.3 VO 4 . 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 greenschist-facies conditions at ultra-oxidized conditions (ᐃFMQ ≥ +16 log units), which are often observed in Mn-oxide ores. Wakefieldite is an efective 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.

american mineralogist

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