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