VARIAT ION IN CLAY MINERAL X-RAY DIFFRACTION RESULTS WITH THE QUANTITY OF SAMPLE MOUNTED 1

ABSTRACT: Two mounting techniques which employ settling of clay minerals through aqueous olutions, show variations in calculated clay mineral abundances which are directly related to the quantity of sample mounted. Hence, for different mounts of a single sediment sample, calculated values for montmorillonite (17 A) range from 0-23%, while kaolinite (7 A) and illite (10 A)vary from 73-55 % and 27-22%, respectively. The change in mont-morillonite content is a real artifact of the quantity of sample mounted, whereas fluctuations in the kaolinite and illite values are the result of calculations in a closed number system and the variable montmorillonite concentration. Other mounting techniques, which do not depend upon particle settling, do not show this pattern. It is suggested that two factors produce the observed effect. The small size of mont-morillonite particles, and their resulting low " settling velocities, result in a segregation of montmorillonite oward the top of the slide-mounted clay film and an enrichment of illite and kaolinite toward the bottom. For thick mountings, this pattern gives excessively high values for montmorillonite. For thinner mountings, the smaller size of montmorillonite par-ticles (compared to illite and kaolinite) makes it impossible for this mineral to achieve a sufficient degree of preferred basal orientation at very low concentrations, and thus the montmorillonite concentration is underestimated. Mounting techniques which employ gravitational settling of particles in suspension, give neither true nor consistent results. Therefore, such techniques hould not be used. I NTRODUCTION Several mounting techniques /or quantitative clay mineral X-ray diffraction analysis have been developed in the past 30 years (Grim, 1934