The role of biological agents in the microstructural and mineralogical transformations in aluminium lateritic deposit in Central Brazil.

Petrological studies using X-ray diffraction (XRD) and micromorphological analyses (Optical Microscopy and SEM), were done to understand the role of biological activity in the evolution of Barro Alto bauxite. The results indicated that this influence came through structural (or microstructural) and mineralogical transformations, namely: I ? the bioturbation caused by termites and II ? themechanical degradation and geochemical transformation promoted by roots. In the bioturbation caused by termiteswere formed: I ? a intergrainmicro-aggregate structure, characterised by gibbsite crystals from isalteritic bauxite fragmentation on the bottom of the profile and II ? a granular structure characterised by a termitic microaggregates with fragments of gibbsite immersed in a kaolinite?gibbsite?goethite?boehmite micromass, formed by bioturbation of the degradation clay with nodules of gibbsite,whose origin is the geochemistry degradation of isalteritic bauxite. The processes associatedwith geochemical andmechanical degradation caused by roots were responsible for the genesis of: I?a porphyric texture with bauxite fragments surrounded by nonaggregate material and II ? fine monic structure where the gibbsite neoformation has the mould cavities left by old roots, generating riziform features

A comparison of properties of clay minerals in isalteritic and in degraded facies.

The mineralogical, geochemical and micromorphological features of an isalteritic clay facies, which originated from weathering of an anorthosite, were compared to those of clay facies derived from the degradation of a bauxite developed from the same rock. The isalteritic clay was formed by the hydrolytic alteration of plagioclase, whereas the degraded clays were formed by decomposition of gibbsite and neoformation of kaolinite. This resilification process resulted from the reintroduction of silica via the oscillation of the phreatic level and/or the decomposition of organic matter on the surface. The degradation process was gradual and yielded two different facies: (a) degraded clays with almost total decomposition of gibbsite, and (b) degraded clays with gibbsite nodules. Morphologically, the isalteritic clays differ from the degraded clays because they contain larger hexagonal and pseudo-hexagonal crystals. The degraded clays have more irregular crystal shapes, ranging from laths to anhedral shapes