Regional differences in clay mineralogy are found in British Lower Palaeozoic slate belts formed during Caledonian terrane amalgamation. Extensional basins in Wales, the northern Lake District and the Isle of Man are characterized by a greater diversity of species in clay mineral assemblages. Slates that evolved in these basins contain both the K- and Na-rich products of the 2:1 dioctahedral reaction series. Pyrophyllite, rectorite and corrensite are sporadically distributed but kaolinite is rarely recorded even in lowest-grade mudstones. In contrast, clay assemblages that evolved in the convergent basins of the Scottish Southern Uplands and northern England generally contain fewer mineral species, and Na-micas and pyrophyllite are rare or absent. Na-bearing clays may have been generated from low-temperature mixing of hydrothermal fluids and seawater in the extensional basins. Such fluids appear to have been unavailable in the convergent basins through lack of volcanic activity in the early stages of basin development.