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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)

Abstract


Journal of Sedimentary Research, Section A: Sedimentary Petrology and Processes
Vol. 64A (1994)No. 3. (July), Pages 694-697

A Method for the Determination of Nitrogen in Clays, with Application to the Burial Diagenesis of Shales: RESEARCH METHOD PAPER

Paul A. Schroeder (1), Ellery D. Ingall (2)

ABSTRACT

A new operationally defined method for the measurement of fixed inorganic nitrogen (Nfix) associated with clays has been tested and applied to a sequence of mixed-layer illite/smectite (I/S) (ranging from 40 to 68% illite in I/S) in shales from a drill site in Terrebonne Parish, Louisiana. This new method has distinct advantages over other methods in that it is simple and can accommodate small sample sizes (10 mg). Nfix is measured, using a carbon-nitrogen analyzer, on a sample that has been ashed for 8 hr at 450°C to remove organic and non-fixed inorganic nitrogen. Total nitrogen is measured on an separate untreated sample. Content of organic carbon and nitrogen can be assessed by the weight-percent difference between untreated find ashed samples. Fourier-tr nsform infrared spectroscopy confirmed the presence of nitrogen as ammonium in untreated and heat-treated samples.

Nfix content of the < 0.2 µm fraction of the shale sequence correlates positively (R2 = 0.91) with the percent of illite in the mixed-layer I/S. Corresponding with the increase in percent of illite in I/S with depth, an increase in Nfix content is also seen with increasing depth of burial. Nitrogen from thermal breakdown of organic-matter during later diagenesis probably provides the Nfix found in I/S. In the range of diagenesis studied here, Nfix concentration appears to depend on the available fixing capacity of illite. Because nitrogen content varies with organic-matter type, however, caution should be used when assuming this relationship for other diagenetic environments.


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