360 CLAY MINERALS Table Typical values for cation exchange capacities Cation exchange capacities (CEC) in milliequivalents/gram (Data from multiple sources.) Kaolinite Halloysite Chlorite Illite Montmorillonite Vermiculite meq/100 g to 18 to 40 10 to 40 10 to 40 60 to 150 100 to 215 (Table 2) Because of their fine particle size, clay minerals are not easily identified by optical methods, though the distinctive chemistry (and sometimes habit or morphology) of most allows identification by X-ray analysis in electron microscope studies The most reliable method of clay identification, particularly in very fine grained rocks is X-ray diffraction, either of the bulk sample, or more reliably, of the fine fraction (usually