The clays and other minerals in a succession of Late Permian coals of high-volatile bituminous to semi-anthracite rank have been identified, using low-temperature oxygen plasma ashing and X-ray diffraction, and evaluated to identify the relative roles in mineral matter formation of detrital input, early diagenesis in the peat swamp and late diagenesis associated with rank advance. Although well-ordered kaolinite of probable early diagenetic origin is abundant throughout the succession, the uppermost and lowermost seams of the sequence, regardless of rank, contain relatively abundant illite and/or interstratified illite/smectite, along with a small but significant proportion of chlorite. These clays are thought to be essentially of detrital origin, washed or blown into the peat deposit in relative abundance during the establishment and subsequent overwhelming of an extensive and long-lived swampy environment. Quartz is also abundant in the lower seams of the sequence, especially close to the regional sediment source area. Illite is unusually abundant in the topmost seam in both high- and low-rank parts of the succession, and thus appears to represent detrital input from a particular source material. Although significant changes are reported in the clays of the associated strata due to rank advance, the principal effect of rank advance on the minerals in the coal itself appears to be the development of an ammonium illite, and possibly some additional fine-grained chlorite, in the semi-anthracite material. Isolation within the organic matter of the coal is thought to have inhibited access for ions such as K+, which might otherwise have become involved in metamorphic reactions and given rise to mineralogical changes commonly found in non-coal sedimentary successions.