A general expression for diffracted intensities from partially ordered layer structures with a 'mistake' has been calculated. The 'mistake' consists of a shift of a layer parallel to adjacent layers by any arbitrary fraction b/q along the b axis, q being any integer. The expression is free from any simplifying approximations.It is well known that layer lattices are much prone to stacking disorder due to their weak interlayer forces. One stacking disorder which frequently occurs in partially ordered layer silicate minerals is a layer shift. This consists of a shift of layers parallel to adjacent layers by nb/3 along the b-axis and is found in many silicate minerals, especially kaolinite and chlorite. The high probability of occurrence of this defect in these minerals has been explained by Brindley (196 l) on the basis of the geometry and structure of the layers. This author argued that layer displacement of exactly 89 in these partially disordered crystals is rather improbable, describing several examples showing that the shift in layer silicates should be an arbitrary fraction of 'b' or 'a' rather than exactly nb/3. Wilson (1962) considered only two specific cases, viz. 89 and 89 and derived intensity expressions for these cases under certain approximations. The first approximation is the conversion of the summation into an integration, while the second one concerns Jm, i.e. j,~ -F2(1 _ fl)m ~. F 2 e-m~. These are valid for large values of N and small values of ft.

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