Diffraction by crystals with planar faults. III. Structure analyses using microtwins

The general theory for kinematical diffraction from crystals having planar faults is applied to the case of microtwinning and the related cases occurring in some minerals where there is a disordered sequence of two types of structure having small differences in composition, unit-cell dimensions and axial orientations. It is shown that, if intensities are measured for unresolved or partially resolved pairs of diffraction spots using conventional techniques, errors may well arise if the measurements are interpreted on the usual assumption that the intensities from the different crystal regions are summed incoherently. Calculations for representative cases suggest that errors, due to the neglect of the spreading of intensity maxima into continuous streaks, may amount to 20 or 30% when overlapping diffraction spots have structure amplitudes of opposite sign, but are usually much smaller, especially if the structure amplitudes are of the same sign.