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Submicroscopy of Clay Microaggregates in an Oxisol from Pernambuco, Brazil
Author(s) -
Santos M. C. D.,
Ribeiro M. R.,
Mermut A. R.
Publication year - 1989
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1989.03615995005300060047x
Subject(s) - clay minerals , kaolinite , organic matter , oxisol , ferrihydrite , nontronite , maghemite , chemistry , mineralogy , geology , chemical engineering , hematite , soil science , soil water , organic chemistry , adsorption , engineering
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.