An x-ray method is developed for identifying chlorite species in fine-grained materials. A survey of the crystal chemistry of chlorites shows that in orthochlorites the principal compositional variables are the Al ions in tetrahedral coordination and the Fe²⁺ ions in octahedral coordination. These can be estimated from the basal (001) spacing and the structure factors of the lower order reflections. The problems which arise with leptochlorites and the applicability of the method to chlorites containing dioctahedral layers are discussed. Four examples are given comparing results obtained by the x-ray method with the results of chemical analysis.