Crystal chemistry of fluorthalenite-(Y) and gadolinite-(Y) from white cloud pegmatite, Jefferson County, CO, USA

Reviews and Highlights	Quantum Science	Molecular and Soft-matter	Ultrafast Nano-optics and Nanophotonics	Mineralogy and Geochemistry

Joseph R. Smyth, Markus B. Raschke, Julien M. Allaz, Evan Anderson, Philip M. Persson, and Radek Koda
Geol. Soc. Am. Abstracts 46, 751 (2014).
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The White Cloud pegmatite is a typical example of the NYF (Niobium-Yttrium-Fluorine) family, located in the South Platte District, Jefferson County, CO, USA. In addition to major quartz, K-feldspar, biotite, and fluorite, the pegmatite exhibits an unusual mineral assembly with several rare earth element (REE) rich minerals including fluorthalenite-(Y), gadolinite-(Y), allanite-(Ce), bastnasite-(Ce), and synchysite-(Ce) closely associated with fluorite. Chemical composition of these minerals was analyzed using wavelength dispersive X-ray spectroscopy (WDS). In order to understand the crystal structure controls on REE distributions, the crystal structures of fluorthalenite and gadolinite were refined using single-crystal X-ray diffraction and site occupancies were determined.

Gadolinite-(Y), nominally (Y,REE)₂Be₂FeSi₂O₁₀, is found to be nearly free of F and OH. The crystal structure (a = 4.839Å; b = 7.731Å; c = 10.182Å; beta = 90.16 deg) has been refined in space group P2₁/cto R = 0.048. The structure has a single 8-coordinated Y/REE site with an enrichment in MREE (Gd, Dy) of about 20 to 25% (apfu). Be and Si are in distinct tetrahedral sites, but site occupancy refinements indicate up to 25% disorder of Be and Si over the two sites. Site occupancy of the Fe site is refined to about 85% using Fe only.

Fluorthalenite-(Y), nominally (Y,REE)₃Si₃O₁₀(F), is HREE enriched and characterized by full occupancy of the fluorine site with F. The crystal structure (a = 7.346Å; b = 11.194Å; c = 10.412Å; beta = 97.34 deg) has been refined in space group P2₁/n to R = 0.048 using 6439 measured intensities. WDS analysis shows strong enrichment in HREE and variation in REE/(Y+REE) ranging from 0.05 to 0.25. For the structural purpose the REE are modeled as Yb. The structure has three distinct 6- or 7- coordinated Y/REE sites each containing 15-20% REE and with a significant enrichment of REE in the Y2 site. REE distributions between gadolinite and fluorthalenite thus appear to be dependent on cation site sizes.