Unlike mineral cleavage , which takes place parallel to planes of atoms within a single crystal, slaty cleavage occurs parallel to and between crystals of flat minerals such as micas or clays. In very fine-grained metamorphic rocks, typically those that were originally shales, mudstones, or tuffs, breakage occurs along flat surfaces separated by distances of microscopic dimensions. As a result, rocks with this style of foliation tend to splinter into flakes or thin sheets.
Foliation develops in response to directed stress during metamorphism.
Generally, new planar features such as slaty cleavage form at an angle to pre-existing planar features such as bedding. This is apparent in the left-hand photo above. This cylinder of rock was cut in a quarry in Minnesota. The dark lines on the side of the cylinder are bedding planes. The top and bottom of the cylinder are cleavage surfaces, not parallel to bedding but at about a 30 degree angle.