Abstract
Clays portions (0.2μ>) separated from both a surface layer and a subsoil of a calcareous paddy field have been investigated. The results of the investigations by chemical analyses and electron-micrographs were reported in a previous article. In the present paper, the same clays are studied through dehydration methods, differ(e)ntial thermal analyses, and x-ray diffraction methods The dehydration curves obtained are like to that of beidellite although in the portion of high temperature over 400℃ the weight loss is higher to some extent than beidellite and the general tendency of the curve is more or less parallel to illite. The moles of water loss above 400℃ is calculated on the basis of per unit cell. The value which corresponds to the OH in the lattice is 2.5 moles of water per unit cell and this supports the conclusion from chemical analyses in the previous paper that there may be five (OH) group in one lattice unit. The differential thermal curves of both clays from the surface layer and the subsoil show two endothermic peaks at 150℃ and 530℃ and exothermic peak at 850℃. These curves are like to the curves of nontronite-type clays. However, the X-ray patterns are nearly equal to that of Fe-montmorillonite. From these investigations the authors conclude that the clays finer than 0.2μ are nontronites of beidellitic, some illitic, and Fe-montmorillonite-type. The conditions forming these clays arese attributed by the authors to a high pH, to a high saturation with bases especially to Ca, to an impermeable character of the profile, and to a high Fe^<++> ion supply in a paddy field in cropping periods.
