Oxidation of Chlorites in Soil Clays and Effect on DTA Curves

Abstract

IN general, the characteristic differential thermal analysis (DTA) curves of trioctahedral 14 A chlorites comprise three peaks: a single relatively large endothermic peak at about 450°–650° C produced by dehydroxylation of the hydroxide layer, a smaller endothermic peak, at about 700° C, due to dehydroxylation of the 2 :1 layer and an exothermic peak, between 750° and 900° C, due to decomposition of the mineral and recrystallization1,2. Mackenzie3 has shown that the DTA curves of chloritic clay fractions from soils seldom show these characteristic peaks and a recent investigation of some chlorite-rich soils in Scotland has confirmed this observation for clay fractions separated from freely drained soils (D. C. B., unpublished data). It was found that the DTA curves of chloritic clays from poorly drained gley soils, in which reducing conditions prevail for most of the time, did show these characteristic peaks. This difference in thermal behaviour is due to the fact that ferrous iron in the chlorite in freely drained soils has been oxidized.