ABSTRACT

A nationally or internationally accepted methodology in semi-quantitative clay mineral studies is lacking. Recent U. S. published papers (1954-1967) on clay mineral contents and distributions in sediments and sedimentary rocks indicate that at least six techniques are being used to make percentage calculations. In addition, there is no consistency in sample preparation methods between laboratories. These diversities only compound the problems already inherent in any clay mineral investigation.

Percentages of the clay minerals in a complex assemblage present in twenty-five samples are calculated by five different methods, but using the same diffractograms. No uniformity is apparent in the results except for the dominance of montmorillonite, which could be inferred from the physical aspects of the samples.

The trends in the relative amount of montmorillonite, as determined by the five different calculation methods, show agreement 83 percent of the time. A random matching for trends would show an agreement 69 percent of the time. Thus, the methods used to predict trends are better than random matching.

A hierarchial design was used to test the importance of slide preparation techniques, subsampling, and calculation methods. Preparation technique is significant at the 5 percent level but not at the 1 percent. Subsampling was not significant at any level while method of calculation was significant at all levels.

We do not give preference to any of the calculation techniques being used because each seems to be as well founded as the others. In the present state of clay mineral studies, accurate quantitative results are unattainable although precision may be good.

Until true quantification becomes possible, we suggest the adoption of a unified methodology, possibly under the aegis of the Clay Minerals Society, so that semi-quantitative clay mineral investigations in sediments and sedimentary rocks can become more scientifically meaningful. Without a semblance of standardization of methods (calculation and preparation), regional studies will remain as at present, incomparable.