Submicroscopy of Clay Microaggregates in an Oxisol from Pernambuco, Brazil

Abstract

Ultrathin sections were made from dehydrated, unoriented but undisturbed soil microaggregates (pseudosands, about 10–100 µm in size) of an Oxisol occurring in the semiarid region of Pernambuco State, Brazil. Objectives were to determine the nature and arrangements of individual particles that comprise the microaggregates. The dominant clay mineral in the microaggregate is poorly crystalline (irregular, pseudohexagonal in shape), densely packed, randomly oriented kaolinite with an average diameter of 0.2 µm. Poorly crystalline Fe oxyhydroxides, probably ferrihydrite, exist as individual aggregates filling the space between the kaolinite particles. Halloysite and Fe‐Ti‐bearing minerals also occur as accessory minerals. An attempt was made to localize organic matter by ultracytochemical reagents. Cell walls, humified hyphae, and other decomposed organic materials, which were easily identified in large pore spaces, appear to play an important role binding the clay particles in microaggregates. Concentrations of organic matter in the magnetic fraction of clays suggests that part of the humified organic matter may be locked up in organo‐Fe complexes. Ultrathin section analyses suggest that the combination of random orientation of clay particles, organic matter, and aggregates of Fe‐bearing minerals, account for the isotropic nature of the aggregates.

Origin of microaggregates in soils with ferralic horizons

“…Various hypotheses have been presented about the geochemical formation of these microaggregates. These can be formed by a simple rearrangement of the plasma (Muller, 1977) or by geochemical evolution that results from physico-chemical interactions between iron and kaolinite (Pedro et al, 1976;Chauvel et al, 1978;Cambier, 1986;Pedro, 1987;Dexter, 1988;Santos et al, 1989). The presence of a domi- nant kaolinitic clay mineralogy ( Figure 4, Table 3), high iron contents in the soil matrix (Table 4) and of evidences of a kaolinite-iron oxide system (Cambier, 1986) on this toposequence suggests that part of these microaggregates could have been formed by geochemical evolution.…”

“…Consequently, the formation of the microaggregate structure can be considered complex due to the existence of various hypotheses that may explain their formation and shape. Among them, a biological origin of microaggregates (Stoops, 1983;Eschenbrenner, 1986;Miklós, 1992;Vidal-Torrado, 1994), a geochemical origin (Chauvel et al, 1978;Cambier, 1986;Pedro, 1987;Santos et al, 1989) and finally a physical origin by fragmentation (Muller, 1977;Trapnell & Webster, 1986) may be involved.…”

Mineralogia, morfologia e análise microscópica de solos do bioma cerrado

“…A subdominância de microagregados em Latossolos de textura média (Quadro 7) já tinha sido mencionada por Lima (1988), com o plasma formando preferivelmente massas aderidas aos grãos que dominam o fundo matricial, embora possam ocorrer Latossolos de textura média mais próxima aos limites da textura argilosa com predominância de microagregados, como aquele estudado por Santos et al (1989). Dessa forma, as diferentes mineralogias não produziram grandes diferenças de microestrutura, bem como já não tinham apresentado na estrutura (Quadro 2), sendo excedidas pelo efeito da textura dos diferentes solos.…”

Relações material de origem / solo e pedogênese em uma seqüência de solos predominantemente argilosos e Latossólicos sobre psamitos na depressão periférica Paulista: Paulo State Peripheral Depression, southeastern Brazil

“…A maioria desses solos situa-se sobre velhas e estáveis superfícies geomórficas (Buol et al, 1980). Muitos consideram que as suas características são resultado de uma pedogênese policíclica, iniciada em sedimentos situados relativamente próximos de suas fontes e depositados depois de previamente intemperizados (Stoops, 1989(Stoops, , 1997Santos et al, 1989;Muggler, 1998). Outros consideram esses materiais provenientes da profunda alteração de material similar ao imediatamente subjacente (Millot, 1977), com ocorrências de remanejamentos localizados e grandemente influenciados por remontes verticais causados pela biopedoturbação (Eschenbrenner, 1996), sem que tenha havido necessariamente um tempo relativamente longo de formação do solum (Miklós, 1992).…”