Summary: Differential thermal analysis alone appears inadequate as a tool for distinguishing the many chlorite varieties; however, this process is capable of yielding much valuable information about the chlorites. The 7 Å septechlorite structure may be easily distinguished from the normal 14 Å chlorite structure. Two stages of dehydration, oxidation of ferrous iron, and structural collapse appear as endothermic and exothermic peaks on the D.T.A. chlorite curves. Septechlorites show only one, large dehydration stage, which leads directly to the structural breakdown. Iron oxidation accompanies the initial dehydration in both structural types, and the temperature at which dehydration begins is noticeably lowered by increasing ferrous iron content of the chlorite variety.