Chemical, Mineralogical and Micromorphological Properties of Glaebules in Some Tropical Lowland Soils

Abstract

The chemical and mineral composition, and micromorphological properties of several kinds of glaebules collected from padi soils in South and Southeast Asian countries were studied. They were separated into ferruginous, manganiferous and carbonate glaebules, in terms of chemical composition. Silica content decreased in glaebules in comparison with that of the padi soils of the area, while the content of bases remained at almost the same level in glaebules and soils. Alumina, titania, and the molar ratio TiO_2/(Al)_2O_3 also stayed at the same level in both. These results suggest that clay in the vicinity of the precipitation center was incorporated into glaebules, while sand grains were excluded. The relative concentration of iron to manganese was correlated with the SiO_2/(Al)_2O_3 ratio, which could be an index of the degree of weathering. This result is compatible with observations that ferruginous glaebules are frequently observed on highly leached soils on older terraces. The most common iron mineral was goethite except in glaebules with a laterite fabric, in which haematite predominates. The crystalline structure of iron minerals estimated by X-ray diffraction appeared to be well correlated with a ratio nearly equivalent to the "Aktivitatsgrad". Well crystallized goethite and haematite resisted Mehra-Jackson's deferration treatment. The clay mineral composition of many glaebules was more or less similar to that of the matrix soil, but the discrepancy was considerable for glaebules thought to have been transported. Laterite fabric is characterized by the growth of iron oxides into coarse granules, such granulated areas being connected or demarcated by channels filled with birefringent clay. Several ferruginous glaebules with laterite fabric were found among the samples from India and Northeast Thailand. They were embedded in the soils by physical transportation of laterite fragments, and underwent subsequent transformation of fabric as exemplified by a thin concentric coating, or by repetization of the outer part. The manganiferous glaebules have clearly developed concentric laminae, indicating the alternate precipitation of manganese and iron oxides in situ. The various glaebules were classified into two groups by their properties, with the aim of distinguishing between autochthonous and allochthonous formations.