ELECTRON-MICROPROBE AND ELECTRON-MICROSCOPE STUDIES OF SOIL CLAY PARTICLES

Several soils were examined in detail by electron microscope and microprobe to assess the nature of clay size particles and microaggregates. Morphology, analytical composition, identity and association between particles were determined. Of the mineral oxides, pellets of goethite and hematite about 1 jum in size were most distinctive. They were found in all the soils examined except one. They contained both aluminium and silicon, sometimes in fairly constant proportions, but the presence of adhering fine material made it difficult to assess the extent to which these elements were part of the iron oxide structure. Discrete gibbsite particles of micron size were rarely encountered, although some soils contained appreciable quantities of it in much smaller form. At least a few rutile or anatase particles were found in all soils, while they were common in some. They also were usually contaminated by surface deposits, in contrast to ilmenite and quartz grains which were always relatively clean and were often isolated from other particles. The ratios of iron/titanium in ilmenites were well defined but different from soil to soil. Uncontaminated clay mineral particles were rare. Most had substantial deposits of fine-grained material upon their surfaces, much of it < 200 Å in diameter. The most easily detected components of these surface deposits were goethite and hematite, and often they were dominant. Finely divided clay minerals were usually present as well, together in some soils with gibbsite and occasionally with titanium oxides. In one soil where coarser (1 μm) host particles were not readily available to carry it, the fine-grained material formed microaggregates consisting of kaolin, gibbsite and hematite, all of comparable size. Soils containing both kaolin and illite were composed mainly of compact microaggregates of the two clay minerals, which had the appearance of single particles. These results demonstrate the close association frequently existing between micron and submicron soil particles and between submicron particles themselves. The structural units of soil particles were often much smaller than generally supposed. © 1979, CSIRO. All rights reserved.