Journal
APPLIED GEOCHEMISTRY
Volume 19, Issue 5, Pages 737-745Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.apgeochem.2003.10.008
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The authors report results from an experimental study on mixtures of pure endmembers of natural clay and carbonate. The scientific rationale is an evaluation as to what extent B contents and B isotopes of carbonate samples may be obscured as a result of contamination with clay, particularly since both authigenic carbonates and biogenic carbonates (e.g. microfossil tests) often contain some clay. Three aliquots of a series of samples (each containing 0, 20, 40, 60, 80, 100% clay) were analyzed. Set I was washed with distilled, de-ionized water; for set 2 the HCl soluble parts were dissolved in 2 M HCl after washing; set 3 was completely digested with 30M HF prior to a series of ion exchanges. Isotope data of the endmembers are 6.6parts per thousand. (100% marble) and -4.6parts per thousand (100% clay), with the clay being the dominant B source (ca. 50 ppm compared with < 2 ppm in the carbonate). For set 1, only the B adsorbed to the clay was washed out with the de-ionized water (δB-11(absorbed) = 12.9-14. 1&PTSTHOUSND; ± 0. 5&PTSTHOUSND;), while no B was mobilized from the carbonate. The HCl-dissolvable B in washed samples of set 2 show slightly increasing B contents with higher clay contents, suggesting that dissolution of the marble as well as B mobilization from the clay account for this trend. δB-11 isotopes tend towards more negative values when clay content increases, indicating that some structurally-bound B is lost from the sheets of linked (Si, Al)O-4 tetrahedra of the clay mineral. This result shows that not only B adsorption, but possibly diffusion or weathering of broken edges of clay minerals releases some structurally bound B of clay minerals. Set 3, where bulk samples were completely HF-digested, shows as expected a linear increase in B concentrations and decreasing δB-11 ratios with increasing clay content. The overall results suggest that relatively small amounts of clay (e.g. as contamination in a microfossil test) have no significant impact on the B content and δB-11 measured for the carbonate, but that care has to be taken if clay exceeds 10wt.% (e.g. carbonate concretions, chimneys, etc.). © 2003 Elsevier Ltd. All rights reserved.
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