Vera de Melo Marcelino

Weathering: Toward a fractal quantifying

Weathering occurs over a wide range of scales. To link features through these scales is a major challenge for interdisciplinary weathering studies. Fractal approach seems to be specially useful for this purpose. We introduce a multistep fractal weathering assessment scheme devoted to extract fractal weathering classifiers from texture analysis of the minerals image. Our scheme enables to quantitatively estimate the global and local information about the geometry of the weathering pattern. This information is basic to develop geometrical indices of weathering, which can significantly enrich the common qualitative and semiquantitative weathering assessment schemes. To justify the fractal approach, a strong statistical self-similarity has been documented for both the weathering and fresh features of two common silica minerals: quartz and biogenic A-opal (phytolith) over four orders of length scales. The procedure is fast, drastically reduces thresholding bias, promises to be universal, it is valid for genetically different minerals and rock types, scale independent, and specially useful for monitoring the changes in the minerals roughness during the alteration. Two of the proposed classifiers seem to be potentially useful for direct application in the field and be used by nonspecialist.

Oxic and Related Materials

Publisher Summary This chapter discusses the oxic and related materials. The oxic horizon is a strongly weathered subsurface horizon with a clay fraction dominated by low-activity clays and iron and aluminium oxides, and with silt and sand fractions mainly composed of quartz and including little or no weatherable minerals. The oxic horizon essentially corresponds to the ferralic horizon of the World Reference Base for soil resources. Oxic and ferralic horizons are diagnostic horizons of Oxisols and Ferralsols, respectively. Micromorphological techniques have been often used in the study of oxic and related materials. Wide attention has been given to the study of the granular microstructure that is typical for these materials, but no consensus has been reached on its origin. Submicroscopic techniques were mainly used to study mineral weathering, mainly of quartz, and the formation of kaolinite, iron oxides, and gibbsite occurrences.

Lateritic and bauxitic materials

Abstract Lateritic profiles are very complex and mostly polygenetic. They have been widely studied but no generally accepted definition or classification of laterite exists. This chapter presents a review of the various micromorphological characteristics of lateritic and bauxitic materials, without following a specific genetic model. A standard lateritic profile includes several layers, from bottom to top: the soft laterite (mottled clay zone), the hard laterite (laterite sensu strictum for some authors), and the gravel-rich layer. The soft laterite is characterised by redistribution of hematite, mostly forming a network of hypocoatings surrounding and isolating kaolinitic cells. The hard laterite is either nodular or vermicular, with high concentrations of hematite and goethite and with common corrosion of quartz grains. Nodules and channels are generally coated by goethite-kaolinite coatings, whose genesis in not well understood. The gravel-rich layer is an accumulation of allochtonous iron oxide nodules with various internal fabrics. In bauxites and Al-rich laterites, several types of gibbsite enrichment, partly marked by different crystal size ranges, are observed in thin sections. Although many descriptions of laterites and bauxites have been published in different disciplines, such as geology, pedology and geomorphology, a lack of coherent concepts and terminology greatly complicates attaining a comprehensive view of their micromorphological characteristics.

Micromorphology of soils of the humid tropics

Abstract Stoops, G., Marcelino, V., Zauyah, S. and Maas, A., 1994. Micromorphology of soils of the humid tropics. In: A.J. Ringrose-Voase and G.S. Humphreys (Editors), Soil Micromorphology: Studies in Management and Genesis. Proc. IX Int. Working Meeting on Soil Micromorphology, Townsville, Australia, July 1992. Developments in Soil Science 22, Elsevier, Amsterdam, pp. 1–15.