Abstract: A standardless method of energy dispersive X-ray fluorescence in conjunction with scanning electron microscopy was used to analyze selected areas of clay-size particles of talc, pyrophyllite, and kaolinite supported by a carbon planchet. Peak intensity ratios of fluorescing elements relative to silicon were converted directly to weight or mole ratios using conversion factors determined theoretically. The conversion factors depend upon particle thickness and mass adsorption coefficients of the sample for the elements analyzed. The effects of particle thickness become significant above ∼0.1 µm. Without using particle thickness corrections, the mean molar ratios of metal to Si agreed to within 6.1, 0.5, and 9.7% of the theoretical ratios for kaolinite, pyrophyllite, and talc, respectively.