Chlorite–mica stacks in the Llandovery sedimentary rocks of central Wales have been studied, both within cleaved turbidite mudrocks, and within early diagenetic apatite, carbonate and monazite concretions. Stacks show a graded distribution within turbidite units, indicating an origin as detrital particles. Concretions provided an arrested diagenetic environment which restricted subsequent deformation and so revealed the nature of the stack precursor minerals. These included detrital mica, and also probable pyroxenes, amphiboles and volcanic rock particles, relic textures of which can be seen preserved within the concretions. Diagenetic alteration to chlorite, probably with some white mica intergrowth, occurred, most probably via smectite and then mixed-layer or swelling chlorite intermediates. Subsequent modification, during the Caledonian cleavage-forming episode, determined the present shape and mineralogy of the stacks. Outside concretions, chlorite pseudomorphs were rotated by tectonic compression, crushed at their margins and pulled apart along (001) cleavage and slip planes. Growth of at least two generations of secondary mica infillings has been recognized, and this was probably accompanied by the growth of secondary chlorite. This resulted in barrel-shaped stacks, elongated parallel with cleavage, and with the (001) plane rotated from the bedding towards the perpendicular to the cleavage. Within early diagenetic concretions it is evident that some stacks had formed prior to lithification. However, these have a relatively simple morphology and the original bedding-parallel grain shapes are largely preserved.