Clay mineralogy of some typical soils in the Brazilian Amazon region

Abstract

The outline of the mineral composition in the clay fraction of some typical soils occurring in the Brazilian Amazon region is as follows: In the soils with oxic B horizon, Yellow Latosols, Red-Yellow Latosols and "Latosols Roxo" Soil, kaolinite is predominant, goethite and haematite are found in general, an gibbsite is frequently detected. A small amount or trace of mica minerals occur in several lateritic soils. The presence of the other 2:1-type clay minerals is rare in the lateritic soils. In Red-Yellow Podzolic soils, kaolinite is also predominant, and iron-oxide minerals are always identified. Considerable amount of gibbsite and small amounts of mica minerals and dioctaliedral vermiculite are present in some of these solis. In fite so-called "Terra Roxa Estruturada" soils, kaolin minerals are dominant, but considerable amounts of iron-oxide minerals are found in the clay fraction, so that haematite is especially rich. Metahalloysite is a dominant clay mineral in a "Terra Roxa Estruturada" Soil. A small amount of dioctahedral vermiculite is present in the other soils. The clay mineral composition of the low land soils is a little complicated. Montmorillonite/dioctahedral-vermiculite interstratified mineral and mica minerals are dominant in two Low-Humic Gley soils and a Ground-Water Laterite soils, respectively, although kaolin minerals are predominant in many lowland soils. Small amounts of 2:1-type clay minerals are frequently detected in the other lowland soils. Quartz exists in the clay fraction of all the soil samples.