The swelling of montmorillonite, in salt solutions and at various water contents, was studied using low angle X-ray diffraction methods. The initial swelling is crystalline and dependent on the hydration energy of the interlayer cation. With monovalent cations interlayer swelling proceeds to the region of macroscopic swelling where the mineral is liquid crystalline. Preliminary calculations, from the experimental data, of the attractive force between neighbouring sheets shows that this force decreases at a lower rate, with respect to distance, than would be expected from van der Waals' forces.

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It was possible to obtain relatively pure goethites (< z p) from a range of soils by dissolving the clay minerals in boiling NaOH. T h e goethites were analysed chemically and studied by X-ray diffraction. All goethites contained aluminium, the amount ranging from 15 to 3 0 mol. per cent. A100H. The ( 1 1 1 ) 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 ( I I I) 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 b y 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 (I I I ) 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. T h e 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. IntroductionEXTENDED observations in this laboratory have shown that the X-ray

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Abstrael--The basal spacing of a set of smectites, with layer charges between 0.74 and 1.14 electrons per unit cell, have been measured while the smectites were in equilibrium with NaCI solutions having concentrations of 0.25 to 2.5 molal. Except for the most highly charged smectites, an expansion from 15.5 to ~ 18.5 ~ occurred as the NaC1 concentrations were reduced. This expansion (or "crystalline swelling") corresponds to the transition from two to three sheets of water between the silicate layers. A random interstratification of 15.5 and 18.5 ~k structure units was present during the change and gave rise to broad diffraction peaks for the 001 reflections. A balance between cation hydration forces, interlamellar electrostatic forces, and van der Waals forces was apparently the basis of the relationship between surface charge and swelling. The results can be expressed in terms of the relative vapor pressures at which the transitions were half complete; these pressures increased with surface density of charge, and over the range of surface charge studied, the P/P~ values ranged from 0.943 to 0.974.