Comparison of Rapid Methods for Chemical Analysis of Milligram Samples of Ultrafine Clays

S. L. Rettig1, J. W. Marinenko2, H. N. Khoury3 and B. F. Jones1
1 U.S. Geological Survey, National Center, MS 432, Reston, Virginia 22092
2 U.S. Geological Survey, National Center, MS 973, Reston, Virginia 22092
3 Department of Geology and Mineralogy, University of Jordan, Amman, Jordan

Abstract: Two rapid methods for the decomposition and chemical analysis of clays were adapted for use with 20–40-mg size samples, typical amounts of ultrafine products (≤0.5-µm diameter) obtained by modern separation methods for clay minerals. The results of these methods were compared with those of “classical” rock analyses. The two methods consisted of mixed lithium metaborate fusion and heated decomposition with HF in a closed vessel. The latter technique was modified to include subsequent evaporation with concentrated H2SO4 and re-solution in HCl, which reduced the interference of the fluoride ion in the determination of Al, Fe, Ca, Mg, Na, and K. Results from the two methods agree sufficiently well with those of the “classical” techniques to minimize error in the calculation of clay mineral structural formulae. Representative maximum variations, in atoms per unit formula of the smectite type based on 22 negative charges, are 0.09 for Si, 0.03 for Al, 0.015 for Fe, 0.07 for Mg, 0.03 for Na, and 0.01 for K.

Key Words: Borate fusion • Chemical analysis • Classic rock analysis • HF decomposition • Microanalytical method

Clays and Clay Minerals; December 1983 v. 31; no. 6; p. 440-446; DOI: 10.1346/CCMN.1983.0310605
© 1983, The Clay Minerals Society
Clay Minerals Society (www.clays.org)