Abstract: Several kaolinite samples were selected for x-ray diffraction studies on the basis of their paper-coating ability. The evaluation of the kaolin clay for paper coating was based on a series of empirical tests including viscosity, resistivity, brightness, and particle-size distribution. The crystallinity of the kaolinites was determined by use of x-ray powder diffraction patterns. In all samples that were good paper coaters, the crystallinity was well developed. Other samples which were poor paper coaters either had poor crystallinity, or, if they had good crystallinity, contained montmorillonite. Crystallinity is not a function of the size fraction. Generally, if the crystallinity is good in the raw sample, then the crystallinity is also good in the < 0.25 micron fraction.
Suitability of kaolinite clay for paper coating varies directly with the perfection of crystallinity and indirectly with the montmorillonite content. Both of these variables can be determined by use of x-ray diffraction data.