Shale is a fine-grained,
clastic sedimentary rock composed of
mud that is a mix of flakes of
clay minerals and tiny fragments (
silt-sized particles) of other minerals, especially
quartz and
calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering or bedding less than one centimeter in thickness, called
fissility.
Mudstones, on the other hand, are similar in composition but do not show the fissility.