D. Robinson writes: The authors have clearly demonstrated in their Fig. 3a the change in shape of the illite basal 10 Å reflection with depth in a weathering profile of the Oxford Clay. They attributed this change to a decrease in crystallinity caused by weathering which results in a progressive loss of interlayer potassium and leads to an increased shoulder on the low 2θ side of the 10 Å peak. Quantification of the shape factor, as the authors suggest, has been widely attempted in clay mineral studies. In the examples quoted (Gill et al. 1977; Weaver 1960) the ratio of the peak-minus-background intensity at 10 Å to that at 10.5 Å is utilized in the examination of crystallinity changes through diagenesis into low-grade metamorphism. This ratio therefore is a measure of the similarity to an ideal 10 Å peak rather than reflecting the importance of the 11 Å d-spacing as in the shape-factor chosen by the authors.

The changes demonstrated in peak-shape, however, could also be caused by the presence of varying proportions of random mixed-layer components in the illite (Thorez 1975). The authors will be aware that there are various pretreatment methods that allow the recognition and semi-quantification of interlayer components (Hower & Mowatt 1966; Weaver 1956). It may be that the various geotechnical parameters could be better understood if there is a varying proportion of interlayer component in the illite, especially if it is of a swelling character.

It is obvious that the authors have many more data

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