Scandium mineralogy : pretulite with scandian zircon and xenotime-(Y) within and apatite-rich oolitic ironstone from Saint-Aubin-des-Châteaux, Armoricain massif, France.
Résumé
The scandium phosphate pretulite has been identified with scandian zircon and xenotime-(Y) in an apatite-rich oolitic Ordovician ironstone at Saint-Aubin-des-Châteaux, Armorican Massif, France. Pseudo-octahedral crystals of pretulite, up to 400 µm across, have grown epitactically on detrital zircon. They reveal complex zoning due to incorporation of Y and HREE, as well as to an extended solid-solution toward the zircon end-member. Characteristic compositions in the pretulite - xenotime-(Y) - zircon system are: Prl0.973Xnt0.020Zrn0.007, Prl0.907Xnt0.088Zrn0.005, Prl0.873Xnt0.042Zrn0.085, Prl0.718Xnt0.024Zrn0.258 and Prl0.453Xnt0.042Zrn0.505. A single-crystal X-ray refinement of the structure in space group I41/amd (R = 0.0389) gives a 6.5870(9), c 5.809(1) Å, for the formula (Sc0.904Y0.032HREE0.016Zr0.048)(P0.952Si0.048)O4. The Raman spectrum is presented. Detrital zircon shows phosphate-rich metamict zones containing HREE and Sc (up to 3.2 wt.% Sc2O3). Analytical and crystallographic data suggest a complete solid-solution between zircon and pretulite. Xenotime-(Y), also epitactic on zircon, shows distinct stages of crystallization, with a decrease in Y together with an enrichment in the lighter REE and Sc (up to 0.7 wt.% Sc2O3). The scandium minerals at Saint-Aubin reflect the evolution of the iron ore, from sedimentation to diagenesis and metamorphism, followed by multistage hydrothermal leaching and recrystallization. Despite the high concentration of Fe in the environment, this quite unique occurrence of Sc minerals illustrates the high capacity of the phosphate ion to extract scandium and precipitate it as a specific phase, at relatively low-temperature conditions.