ERROR DUE TO SEGREGATION IN QUANTITATIVE CLAY MINERAL X-RAY DIFRACTION MOUNTING TECHNIQUES

Abstract

Experimental study of seven common mounting techniques for quantitative analysis of clays by x-ray diffraction showed an acceptable precision, or reproducibility (about plus or minus 10%) for all techniques. Accuracy, or closeness to true value, however, varied as much as 250% in the four techniques involving particle settling in aqueous solutions. Use of these techniques resulted in a surface segregation of montmorillonite due to its smaller size and therefore lower (1/100) settling velocity than kaolinite or illite. This segregation was demonstrated by (1) analysis of samples with the surface layer removed and (2) top and bottom analysis of samples mounted on x-ray-transparent plastic membrane. The four techniques not acceptable for clay mineral analysis include: centrifuge-on-glass slide and centrifuge-on-ceramic tile techniques; pipette- or dropper-on-glass slide technique; and beaker-on-glass slide technique. The three mounting techniques acceptable for clay mineral analysis with regard to precision and accuracy are: smear-on-glass slide, suction-on-ceramic tile and powder press techniques. /AUTHOR/