A laboratory study was conducted to determine the adequacy of using a 2-M sodium carbonate (Na2CO3) solution to digest radiolarian-rich Eocene and Miocene samples to find their total opal content. In general, this commonly used method is not sufficient for complete digestion, even when digestion times are nearly tripled. However, digestion does proceed to completion when a 2-M potassium hydroxide (KOH) solution is substituted for the Na2CO3 reagent and digestion proceeds for 8.5-10 hr. Except when volcanic ash is present, a 2-M KOH digestion produced more accurate biogenic silica results for Paleogene samples with a high opal content than did a Na2CO3 leach. This conclusion is based on biogenic silica results obtained by an independent method of analysis (normative analysis) for Paleogene radiolarian-rich samples. More importantly, the KOH treatment does not appear to leach "excess silica" from the matrix components (such as clay minerals and other siliciclastics) in sufficient concentrations to compromise the relative accuracy of the results. In contrast, the Na2CO3 leach greatly underestimates the amount of biogenic silica in the Paleogene samples. For samples containing volcanic ash, both treatments overestimate the amount of biogenic silica present, with the KOH treatment producing values two to three times greater than the Na2CO3 leach. The commonly used 2-M Na2CO3 leach is inadequate for the rapid digestion of biogenic opal for radiolarians (e.g., Leg 199 sediments), but a 2-M KOH solution will produce reasonably accurate results.
1Examples
of how to reference the whole or part of this volume can be found under "Citations"
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2Center for Geophysical
Investigation of the Shallow Subsurface, Boise State University, Boise ID 83725,
USA. Correspondence author: mlyle@cgiss.boisestate.edu
Ms 199IR-106