T318 Pegmatites I Have Known and Loved (Posters)

Wednesday, 22 October 2014
9:00 AM - 6:30 PM
Exhibition Hall C

CRYSTAL CHEMISTRY OF FLUORTHALÉNITE-(Y)  AND GADOLINITE-(Y) FROM WHITE CLOUD PEGMATITE, JEFFERSON COUNTY, CO, USA

Paper number: T318-4
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

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 a unusual mineral assembly with several rare earth element (REE) rich minerals including fluorthalénite-(Y), gadolinite-(Y), allanite-(Ce), bastnäsite-(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 fluorthalénite 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 = 90.16º) 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.

Fluorthalénite-(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 = 97.34º) 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 fluorthalénite thus appear to be dependent on cation site sizes.