The key role of micromorphology in studies of the genesis of clay minerals and their associations in soils and its relevance to advances in the philosophy of soil science

Authors: GORDON JOCK CHURCHMAN

Abstract: Micromorphological observations from 3 different published works have been studied to aid understanding of aggregation and of colloids, both unique to soils. Saprolites in Hong Kong included ‘veins’ of different thicknesses and colours. Optical mineralogy identified them as infill from the neogenesis of clays in rock fractures. The common thicker infills resulted from weathering. Dark infill contained comminuted primary minerals whereas thin pale infill originated hydrothermally. Scanning electron microscopy (SEM) showed that the size, shape, and mineralogy of the kaolin minerals formed in infill depended on the types of cracks in the saprolites and on drying. Energy-dispersive X-ray spectroscopy analyses showed Fe and/or Mn in dark-coloured infill from comminution of primary minerals upon brecciation, or else beside pale infill in tuff, showing seasonal drying in tuff but not in granite. Pale infill gave predominantly large tubular halloysite in granite but large platy kaolinite in tuff, except that hydrothermal kaolin gave small particles. In dark infill, kaolin particles were also small and were kaolinite and halloysite mixtures. The effect of impurity Fe and Mn in constraining kaolin mineral crystallinity in infills simulates some of the effects of impure soil environments. Long-term cultivation of soils in Australia led to environmental scanning electron microscope images of large microaggregates indicating their breakdown and loss. Transmission electron micrographs of ultrathin sections showed that microaggregates of clay size, comprising clay minerals and oxides covering other materials, including organic matter, were predominant in virgin soil but were broken down to fine clay particles that blocked pores in cultivated soils. SEM showed a web of biological origin in long-term irrigated sandy New Zealand soil that surrounded macroaggregates but only became closely attached on drying. The nature of the macroaggregates was affected strongly by their history of drying, even during preparation for analyses. Micromorphology is especially useful for indicating the nature of aggregates in situ in soils.

Keywords: Aggregates, microaggregates, macroaggregates, aggregate stability, electron microscopy, colloids, neogenesis

Full Text: PDF