GEOMORPHOLOGY, MINERALOGY AND GENESIS OF FOUR SOILS ON GNEISS IN SOUTHEASTERN BRAZIL

SERVULO BATISTA DE REZENDE, Purdue University

Abstract

One Eutrophic Podzolic and three latosolic soils from the Zona da Mata and the Campos das Vertentes Zones of Minas Gerais State, Brazil were selected to study mineral weathering and soil formation, using a pedogeomorphic approach. The geographic area is hilly to mountainous characterized by a deep weathering mantle and low fertility soils. The soils were sampled from pits and along deep road cuts for chemical, physical and micromorphological analysis. According to Soil Taxonomy, the soils were classified as follows: Latosol-Cambisol (LC), Typic Haplorthox fine loamy, gibbsitic, thermic; Humic Latosol (LH), Haplohumox; Red-Yellow Latosol (LV), Typic Haplorthox, fine-clayey, kaolinitic, thermic; and Red-Yellow Eutrophic Podzolic (PE), Typic Eutrorthox, clayey, kaolinitic, thermic ("Oxic Kandiudult" according to recently proposed changes). According to elevation and landscape position, the relative ages of the land surfaces are: LC (GREATERTHEQ) LH > LV > PE. Soils were closely related to geomorphological evolution; younger soils were not only locally lower down the slope, but they also were regionally lower leveled. Quartz, kaolinite, gibbsite and goethite were the major minerals in the soils; gibbsite was mostly derived from feldspars but some was also derived from kaolinite. Kaolinite was derived mainly from biotite and remained largely as pseudomorphs in C horizons. According to gibbsite/kaolinite ratios, the degree of weathering followed this sequence, LC > LV > LH > PE, for B and upper C horizons, and LC > LH > LV > PE for C3 horizons. The color of B horizons followed the hematite/goethite ratio; soils with a higher ratio had a redder color. The highest hematite/goethite ratio was found in the B3 horizon of the LC soil. All other soils were high in goethite. Hematite/goethite was related to the radiation absorbance-reflectance pattern in the visible 400-490 nm band. High goethite samples had an absorbance minimum at 440 nm and a maximum at 470 nm. A100H replacement in goethite was high but not uniform with depth in the PE, LV and LH soils and ranged from 19 to 28%. In the LC soil that replacement was as low as 2.8%. There was a tendency for an inverse relationship between gibbsite content and aluminum substitution in goethite. In the change from parent rock to the C horizons there were many mineralogical changes, but many of the newly-formed minerals retained the form of parent minerals. Thus, pseudomorphs of kaolinite after biotite and gibbsite after feldspar and concentration of iron oxides at the edges of biotite grains were common. Also, the kind of iron mineral that dominates most of the profile was established at the boundary of parent rock and the C horizon. In the change from C horizon to B horizon there were few mineralogical changes but many important changes in morphology. The clay minerals in the pseudomorphs were homogenized into micropeds and aggregates. Biological activity appeared to be a major process causing these transformations.

Degree

Ph.D.

Subject Area

Mineralogy

COinS