Evaluation of the coprecipitation of incompatible trace elements with fluoride during silicate rock dissolution by acid digestion

Insoluble fluoride precipitates which form during HF digestion of mafic sil icate rocks coprecipitate in their structures the trace elements such as Rb , Sr, Y, Ca, Ba, REE, Pb, Th and U, thus hindering their accurate determina tion. We have estimated quantitatively the coprecipitation of trace element s into such fluorides, and suggest an effective method of digestion that ca n suppress completely fluoride precipitation. Conventional acid digestion o f three samples of mafic and ultramafic silicate rocks resulted in the prec ipitation of sticky material and very poor yields of certain trace element in the resultant solution. XRD analysis indicated that the precipitates wer e composed of fluorides such as CaAlF5, CaMg2Al2F12, Na0.88Mg0.88Al1.12(F,O H)(6). H2O and MgF2, the formation of which depended on the major element c omposition of the rock sample. Coprecipitation of trace elements appeared t o be strongly controlled by both ionic radius and valency of the elements a s well as the species of the host fluoride precipitate, resulting in select ive losses of the elements into these fluorides. On the other hand, almost 100% of the trace elements were recovered using larger amounts of HClO4 tha n is conventionally used and evaporating the sample to dryness in a step-wi se fashion. Using this method, white precipitates were formed as oxides of high field strength elements after decomposition of the sample. Coprecipita tion of trace elements of interest in this study with the oxides is negligi ble except for Th for which 0.5-3.2% by weight was coprecipitated probably as the insoluble oxide. As our method also results in negligible blank valu es, it can be used for both the accurate determination of trace element usi ng ICP-MS as well as isotope analysis using TIMS. (C) 1999 Elsevier Science B.V. All rights reserved.