In order to assess the importance of particle size on elemental composition, structure, morphology, and charge characteristics of 2:1 micas, mechanically produced clay separates were analyzed by; electron probe microanalysis (EPMA), transmission electron microscopy (TEM), thermogravimetry, specific surface area analysis, and ion exchange. Books of biotite and muscovite were reduced to clay size « 2.0 μm) particles by mechanical comminution, then further fractionated into coarse (0.2-2.0 μm), medium (0.08-0.2 μm), and fine (< 0.08 μm) clay sizes. Composition of the clay size micas was particle size dependent, compositional changes being smaller for the medium and coarse clays. Grinding produced significant losses of interlayer K, decreased crystallinity, but increased water content as particle size decreased. The medium and coarse clays maintained coherent basal diffractions, whereas, significant peak broadening of x-ray diffraction maxima was observed for the fine clays. Diffracted peak broadening was due to the reduced crystallite size and the semirandom orientation of lath shaped particles. Specific surface area, adsorbed water. and structurally coordinated water increased as particle size decreased. Specific surface areas were determined by adsorption of molecular nitrogen, at liquid nitrogen temperatures. The increased water contents were measured by thermogravimetric analysis. An increase in CEC of the muscovite clays was detected with decreasing particle size, increasing solution pH, and increasing ionic strength. Assignment of specific exchange capacities for the three clays was confounded by AI hydrolysis and depressed solution pH. / Master of Science