Abstract
Treatment of iron (III) oxides with hot 5 M NaOH solution can selectively dissolve silicate impurities (Kampf and Schwertmann, 1982). In the current work 5M NaOH solutions enriched in 18O (+670%.) were employed to determine if this method could be used to purify goethites for δ18O analysis. These experiments suggest that the structural stoichiometric oxygen in well-crystallized goethites does not exchange with hot 5 M NaOH solution. Therefore, this selective dissolution method appears to be isotopically viable.18O-“normal” 5 M NaOH treatments were applied to goethite-dominated ooids of the Upper Ordovician Neda Fm. ironstone. While not completely removed by successive NaOH treatments, the impurities were incrementally dissolved in constant elemental proportions (within analytical error). Consequently, the δ18O value of the endmember goethite could be determined by material balance calculations. This goethite δ18O value is −1.0%. for all analyzed samples of Neda Fm. ooids, including those from occurrences about 200 km apart. The spatial uniformity of the oolitic goethite δ18O values suggests uniform conditions of goethite formation. The conditions might have been those of a low latitude continental weathering environment.
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