The Mineralogical Composition of Shales

Abstract

ABSTRACT An X-ray diffraction-absorption technique provides quantitative analysis of the quartz, feldspar, and carbonate content of shales. Analysis of artificial mixtures of the nonclay mineral components in a clay matrix show that an accuracy of ±10 percent of the amount present can be reliably obtained. Comparisons were made between the diffraction-absorption technique, a combination chemical-X-ray method, and thin-section analyses. The results suggest that diffraction-absorption and chemical-X-ray methods may be reasonably correlatable, but there is considerable variation between the diffraction-absorption technique and thin-section analysis; thin-section estimates of quartz tend to be lower and small amounts of feldspar (5-20 percent) are greatly underestimated or completely overlo ked. The three hundred shale samples analyzed were chosen so that the over-all average of these analyses is representative of the average mineralogical composition of claystone-type shale. The data derived from these analyses indicate that the average mineralogical composition of shale is: 30.8 percent quartz, 4.5 percent feldspar, 3.6 percent carbonate,