THE ISOMORPHOUS REPLACEMENT OF IRON BY ALUMINIUM IN SOIL GOETHITES

Abstract

SummaryIt was possible to obtain relatively pure goethites (> 2 μ) from a range of soils by dissolving the clay minerals in boiling NaOH. The goethites were analysed chemically and studied by X‐ray diffraction. All goethites contained aluminium, the amount ranging from 15 to 30 mol. per cent. AlOOH. The (III) spacing decreased steadily with increasing aluminium content. A study of different particle sizes from one soil showed that the aluminium content increased with decreasing particle size, and the (III) spacing showed a corresponding decrease. Extraction of soil goethite by free‐iron removal agents, such as dithionite, brought into solution that aluminium contained in the goethites. The ease with which the goethite was dissolved by such treatments was inversely related to the amount of aluminium in the goethite, and led to the surprising situation in one soil where the coarser goethite was more easily extracted than the fine goethite. Measurements of the (III) spacings of many other soil goethites showed that almost invariably the unit cell dimensions were lower than those of pure goethite, indicating aluminium to be a very common replacement for iron in soil goethites. The degree of substitution of aluminium for iron probably depends on the weathering conditions in the soil. The aluminium probably restricts the size to which the goethite crystals can grow; it may, by its influence on particle size and by changing the composition of the goethite, alter the colour of goethite; it could modify the phosphate‐fixing power and other chemical properties of soil goethites.