The Mengyejing potash deposit, which is located in the Simao Basin, Yunnan, contains different clastic rocks, namely greyish-green breccias coexisting with potash veins ("green potash"), brownish-red breccias coexisting with potash veins ("red potash") and yellowish-brown mudstones. XRD, SEM and TEM analyses were performed on those fine-grained clays extracted from clastic components. SEM and TEM observation suggests that those clay minerals are mainly composed of hexagonal, euhedral illites. It is very likely those illites are authigenic origin. The XRD results show that the "red potash" are composed of chlorite and illite, whereas both "green potash" and yellowish-brown mudstone contain "pararectorite" type clays, in addition to chlorite and illite. The Kübler values (indication of illite crystallinity) of the "red potash" range from 0.327 to 0.452°Δ2θ, which are much higher than those of the "green potash" and yellowish-brown mudstone (0.221 to 0.283°Δ2θ), indicating illites from the "green potash" and yellowish-brown mudstones have higher crystallinities than those from the "red potash". The "green potash" and "red potash" derived from the same provenance, and have undergone similar geologic processes. Thus the discrepancies with respect to clay assemblages and crystallinities among different clastic sediments were not merely caused by temperature and pressure. Previous studies have shown that the "green potash" were more reductive, imposed by more bacterial activities and had lower pH values, which are favourable to facilitate the crystallization of illite.