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Variation in Clay Mineral X-Ray Diffraction Results with the Quantity of Sample Mounted
Per R. Stokke (2), Bobb Carson
Two mounting techniques which employ settling of clay minerals through aqueous solutions, 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 montmorillonite 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 montmorillonite particles, and their resulting low settling velocities, result in a segregation of montmorillonite toward 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 particles (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 should not be used.
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