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

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.