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

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doi: 10.1016/S0009-2541(98)00206-X
Authors:Yokoyama, Tetsuya; Makishima, Akio; Nakamura, Eizo
Author Affiliations:Primary:
Okayama University, Pheasant Memorial Laboratory for Geochemistry and Cosmochemistry, Misasa, Japan
Volume Title:Chemical Geology
Source:Chemical Geology, 157(3-4), p.175-187. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0009-2541
Publication Date:1999
Note:In English. Includes an appendix. 35 refs.; illus., incl. 3 tables
Summary:Insoluble fluoride precipitates which form during HF digestion of mafic silicate rocks coprecipitate in their structures the trace elements such as Rb, Sr, Y, Cs, Ba, REE, Pb, Th and U, thus hindering their accurate determination. We have estimated quantitatively the coprecipitation of trace elements into such fluorides, and suggest an effective method of digestion that can suppress completely fluoride precipitation. Conventional acid digestion of three samples of mafic and ultramafic silicate rocks resulted in the precipitation of sticky material and very poor yields of certain trace element in the resultant solution. XRD analysis indicated that the precipitates were composed of fluorides such as CaAlF5, CaMg2Al2F12, Na0.88 Mg0.88 Al1.12(F,OH)6 · H2O and MgF2, the formation of which depended on the major element composition of the rock sample. Coprecipitation of trace elements appeared to be strongly controlled by both ionic radius and valency of the elements as well as the species of the host fluoride precipitate, resulting in selective losses of the elements into these fluorides. On the other hand, almost 100% of the trace elements were recovered using larger amounts of HClO4 than is conventionally used and evaporating the sample to dryness in a step-wise fashion. Using this method, white precipitates were formed as oxides of high field strength elements after decomposition of the sample. Coprecipitation of trace elements of interest in this study with the oxides is negligible 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 values, it can be used for both the accurate determination of trace element using ICP-MS as well as isotope analysis using TIMS. Abstract Copyright (1999) Elsevier, B.V.
Sections:Apparatus and techniques
Subsections:Chemical, XRF, and other instrumental methods
Subjects:Experimental studies; Fluoride ion; Fluorides; Fluorine; Geochemistry; Halides; Halogens; High-field-strength elements; Laboratory studies; Precipitation; Sample preparation; Silicate rocks; Solution; Techniques; Trace elements; X-ray diffraction data
Abstract Numbers:99M/3476
Record ID:1999034213
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands
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