Mineralogical composition of a deep black soil profile of India: cation saturation effect on DTA curves of a smectitic mineral

Abstract

X-ray difffaction and differential thermal (DTA) techniques were applied to evaluate the semi-quantitative mineralogical composition of a deep black soil profile from India. X-ray investigations revealed the predominance fo a smectic mineral throughout the profile. The mineral gave its main dehydroxylation peak between 532°C (Mg-sat.) and 550° (K-sat.), indicating that it belonged to the nontronite-beidellite series. The concentration of this mineral decreased with increasing particle size, i.e. increasing concentration of feldspars and amphiboles. In addition, illite, iron-rich chlorite, quartz, attapulgite, sepiolite and some mixed-layer minerals were detected. The effect of cation (Mg ++, Ca ++, K +) saturation on the intensity, symmetry and temperature of the DTA peaks is discussed. Abnormal behaviour of double dehydroxylation in two clay fractions was found to be due to the abundance of Mg ++ cations in the exchangeble positions K + and Ca ++-saturation of the clays did not produce this effect.