Slate is formed from clay-rich mud in response to tectonic stress.
Here we offer a unifying model that views the mechanisms as a function of the combined thermal and strain energy of the system. This model for slate cleavage has arisen from recent electron microscopy and X-ray work on slates from different, well constrained environments. Optical and scanning electron microscopy can be used to resolve detrital grains, which are about 10 micrometres in size, whereas transmission electron microscopy is needed for authigenic grains, which are about 10 nanometres. To measure the crystallographic orientation of all phyllosilicates in proportion to the volume of a particular population in a specific orientation, X-ray texture goniometry is used. Transitional bedding-cleavage orientations are rarely observed.