Oxygen isotope geochemistry of kaolinite in laterite-forming processes, Manaus, Amazonas, Brazil

Mineralogic, crystallographic, and oxygen isotopic analyses were made of kaolinites from a 20 m thick lateritic profile developed on the detrital sands and clays of the mid-Cretaceous to Tertiary Alter-do-Chao formation, central Amazon Basin, Brazil. A detailed delta 18O profile of soil water from the upper 4 m of the same section was also measured. Kaolinite separates were prepared from different pedogenic facies of small hand specimens. Most of these kaolinite separates exhibit polynomal size distributions. The separates were further divided into individual particle size modes for subsequent isotopic and crystallographic analysis. In general, Hinckley Index and delta 18O values of different size fractions of the same sample differ. Hinckley Index values decrease strongly, and Fe content increases in the upward direction in the profile. We conclude that the kaolinite in the profile has progressively evolved in response to changing microenvironments as weathering fronts have moved progressively downward, different kaolinite fractions, even within a single facies of a single sample a few cm3 in size, typically formed under somewhat different conditions, the bulk of the fine-grained kaolinite in the lateritic profile does not constitute a continuous, inverted time record ; that is, lower levels do not represent more recent kaolinite formation, and higher levels do not represent older kaolinite formation, and isotopic compositions of kaolinites from lateritic paleosols should provide an indication of mean climatic conditions and some measure of seasonality during the interval preceding the removal of the laterite from the active weathering environment... (D'après résumé d'auteur

Relações pedomorfogeológicas nas chapadas elevadas do Distrito Federal Pedomorphogeological relations in the chapadas elevadas of the Distrito Federal, Brazil

A caracterização pedológica e o entendimento das relações entre pedologia, geologia e geomorfologia são importantes para a compreensão da distribuição dos solos numa paisagem. O objetivo deste estudo foi avaliar as relações pedomorfogeológicas na região das Chapadas Elevadas do Distrito Federal (DF), mediante caracterização química, física, mineralógica e geoquímica dos solos de ocorrência nesse compartimento da paisagem. Foram selecionadas duas topossequências representativas da distribuição pedológica nas Chapadas Elevadas do DF, cujos solos foram formados a partir de rochas metassedimentares do Grupo Paranoá, representados por Latossolos Vermelhos (LV), Latossolos Vermelho-Amarelos (LVA) e Cambissolos (C). Os Latossolos das duas topossequências apresentaram a maioria dos atributos físicos, químicos e mineralógicos semelhantes. A variação da cor nesses Latossolos é proveniente da mineralogia diferenciada dos óxidos de Fe - hematita predominante nos LV e goethita nos LVA. No entanto, as análises por meio de ICP-AES apresentaram teores de Fe2O3 similares nos Latossolos, demonstrando material de origem (rochas metassedimentares) de composição geoquímica semelhante. A formação da goethita nos LVA foi considerada dependente da sua posição geomorfológica de desenvolvimento, nas bordas das chapadas, onde a oscilação do lençol freático proporcionou a formação de horizonte litoplíntico, com consequente deficiência das condições de drenagem. Os Cambissolos apresentaram-se quimicamente semelhantes aos Latossolos em razão do material de origem, que são rochas metassedimentares já pré-intemperizadas. O estudo das relações pedomorfogeológicas permitiu constatar que a distribuição dos solos nas Chapadas Elevadas do Distrito Federal é condicionada pela evolução geomorfológica e pela geologia da região.<br>Detailed studies of pedologic characterization, as well as of relationships between pedology, geology and geomorphology are important for the comprehension of the soil distribution in a landscape. The objective of this study was to evaluate the pedomorphogeological relationships in the Chapadas Elevadas of the Distrito Federal, by the chemical, physical, mineralogical and geochemical characterization of the soils found in this landscape compartment. Two representative top-sequences of the pedologic distribution were selected in this geomorphologic unit, whose soils were formed from metasedimentary rocks of the Grupo Paranoá, represented by Red Oxisols (Latosols), Yellow-Red Oxisols and Cambisols. The physical, chemical and mineralogical properties of the Oxisols of the two top-sequences were mostly similar. The color variation in the Oxisols was due to the differentiated Fe oxide mineralogy - predominantly hematite in LV and goethite in LVA. However, the ICP-AES analysis showed similar Fe2O3 levels in Oxisols, indicating a material of origin (metasedimentary rock) with similar geochemical composition. The goethite formation in LVA was related to the geomorphological position of development, on the edges of the plateau, where the water table oscillation resulted in the formation of a lithoplintic horizon, with consequently deficient drainage conditions. The Cambisols were chemically similar to Oxisols, due to the source material, consisting of pre-weathered sedimentary rock. The pedomorphogeological relations showed that the soil distribution in the Chapadas Elevadas of the Distrito Federal is linked to the geomorphologic evolution and regional geology

Mineral weathering in acid saprolites from subtropical, southern Brazil Intemperismo subtropical de minerais em saprolitos ácidos do Sul do Brasil

Because weathering of minerals releases chemical elements into the biogeochemical cycle, characterization of their weathering products helps to better model groundwater quality, formation of secondary minerals and nutrient flux through the trophic chain. Based on microscopic and elemental analyses, weathering of riodacite from Serra Geral formation was characterized and weathering paths proposed. Three weathering paths of plagioclase phenocrystals were identified: plagioclase to gibbsite (Pg1); plagioclase to gels and gibbsite (Pg2); and plagioclase to gels, gibbsite and kaolinite (Pg3). Pyroxenes weathered to smectite and goethite (Py1), or to goethite and gibbsite (Py2), and magnetite weathered directly into iron oxides. Rock matrix comprises 90% of rock volume, and weathered to kaolinite and gibbsite, which explains why these minerals were the most abundant in the weathering products of these saprolites.<br>O intemperismo de minerais primários disponibiliza elementos químicos no ciclo biogeoquímico, que por sua vez influencia a qualidade da água subterrânea, formação de minerais secundários e o fluxo de nutrientes na cadeia trófica. Com base na análise microscópica e elemental, o intemperismo de riodacito da formação Serra Geral foi caracterizado e mecanismos de intemperismo propostos. Três processos principais de intemperismo de fenocristais de plagioclásios foram identificados: plagioclásio para gibbsita (Pg1); plagioclásio para gel e gibbsita (Pg2), e plagioclásio para gel, gibbsita e caulinita (Pg3). Piroxênios intemperizaram-se para esmectita e goetita (Py1), ou para goethita e gibbsita (Py2), e magnetita intemperizou-se para óxidos de ferro. A matriz da rocha compõe 90% do volume total, e intemperiza-se para caulinita e gibbsita, o que explica a abundância destes minerais nos produtos de intemperismo destes saprolitos