FLUORIDE PRECIPITATES IN SILICATE WET-CHEMISTRY - IMPLICATIONS ON REE FRACTIONATION

Precipitates of hieratite, ralstonite and NaUZr2F12 form during HF-HNO3-HClO4 dissolution of dolerite samples. These minerals are responsible for fractionation of Sm and Nd from the solution, giving rise to spurious analytical results. Fractionated Sm and Nd can be recovered by dissolution of hieratite in hot (373 K) water or HCl. The presence of ralstonite and NaUZr2F12 remains a problem, as these phases do not dissolve in acids or in hot water. However, their formation could be prevented by the addition of a complex-forming agent such as boric acid during the dissolution procedure. The use of a m-benzene disulphonic acid and hydrofluoric acid mixture is recommended during the dissolution of silicate rocks and minerals for purposes of routine analysis of trace elements.