On the nature of scaly fabric and scaly clay

Scaly clay, deriving from the Italian argille scagliose, is a term that has been used with a range of meanings, from stratigraphic to genetic, and across many scales of observation. Moreover the diagnostic feature of scaly clay—scaly fabric—has a variety of associated expressions used differently in different geological or structural settings. In an attempt to clarify and rationalise these confused terminologies, we have analysed a wide range of scaly clays of clearly contrasting origin. We here describe the appearance and nature of the fabrics at different scales of observations and interpret the mechanisms responsible for their development. Importantly, mesoscopic similarities may well not be reflected at the microscopic scale. As a result, we recommend that the term scaly fabric should only be used for description at the hand-specimen scale, although the fabric can be sub-classified microscopically according to the shape and arrangement of the rock components. Because scaly fabric defines the tendency of the rock to break along specific surfaces and has a morphological expression, we characterise it as a variety of rock cleavage.