Rare earth elements in silicate systems: II. Interactions of La, Gd, and Yb with halogens

The local geometry of sites occupied by trace amounts (0.2 wt%) of La3+, Gd3+, and Yb3+ was examined as a function of composition in quenched silicate melts containing fluorine or chlorine using Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. The glass compositions examined were albite (NaAlSi3O8), sodium trisilicate (Na2Si3O7), and a peralkaline composition (Na3.3AlSi7O,17) approximately halfway between the two. The volatile contents were approximately 1–2 wt% fluorine or. chlorine. Our results indicate that La, Gd, and Yb form complexes with fluorine in the glasses studied. Gd and Yb are coordinated by eight fluorines with no apparent oxygen neighbors [d(Gd-F) = 2.30Å; d(Yb-F) = 2.23 Å]. La appears to form mixed O-F complexes consisting of eight ligands approximately evenly divided between fluorine and oxygen [d(La-O) = 2.45 Å; d(La-F) = 2.46 Å]. We found no evidence for REE-Cl complexes in the compositions studied. The rare earth sites in chlorine-containing melts are, however, more regular than those in volatile-free melts, suggesting that Cl acts to depolymerize the tetrahedral network. Fluorine, because of complex formation and its effect on melt structure and chlorine, because of its effect on melt structure, should both act to decrease crystal-melt partition coefficients of rare earths.