Back-scattered electron imaging was used in conjunction with energy-dispersive spectral analysis to study phyllosilicate minerals in polished thin-sections of Carboniferous (Westphalian) sandstones and associated mudstones from the East Midlands and from the diagenetically higher grade area of Abernant in the South Wales coalfield. Chemical diagenesis proceeded more rapidly in the sandstones than in the mudstones. In the East Midlands, greater flow of freshwater through the sandstones than the mudstones produced a higher degree of alteration of muscovite to kaolinite in the sandstones. Chlorite is present in the Well 1 and Well 2 East Midlands sandstones but not the mudstones nor any Well 3 samples. The chlorite may be an early diagenetic replacement of biotite. Much chlorite has been replaced by kaolinite. In the Abernant samples chlorite is present in both the sandstones and mudstones. In the former it appears to have replaced detrital biotite. In the latter the chlorite is too fine-grained to discern whether it is detrital or authigenic. The chlorite may have formed during the Hercynian orogeny. Much of the mica and chlorite in the Abernant sandstones has been replaced by illite, possibly in the post-orogenic period. The scarcity of kaolinite in the Abernant samples reflects the lack of freshwater leaching.