Asymmetry of 1 nm XRD reflection and measurement of illite crystallinity

Theoretically the X-ray emission is subjected to the Gaussian distribution and is symmetric. An X-ray diffraction peak should be symmetric, too. However all illite 1 nm (interplanar distance) peaks used for measurement of illite crystallinity (IC) are practically asymmetric. Our experimental results prove that any X-ray diffraction peak in low diffraction angle segment appears asymmetric if the diffractometer is running with a slit-fixed system. However, if the diffractometer is running with an auto-adjustable-slit system and the illumination length is fixed, the X-ray diffraction peak in low diffraction angle segment is symmetric. Those peaks derived from synchrotron radiation are symmetric in all angle ranges. The asymmetric degree (AsD) of a X-ray diffraction peak is subjected to the ratio of integrated intensities on lower and higher diffraction angle sides which are related to the X-ray illuminating length (area) on the sample. From the expression of illuminating length it is derived that with increasing diffraction angle the illuminating length decreases and therefore a X-ray diffraction peak is always asymmetric. The relationship between AsD and IC can be expressed as AsD = 0.239IC + 0.999, When illite/smectite mixed-layer phase presents the asymmetry of the illite 1 nm X-ray diffraction peak will be obviously higher than usual case and induces unusually larger IC value.