Shale

Shale is a fine-grained sedimentary rock formed from mud. The mud 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.^[1] Shale has breaks along thin laminae (plates) or parallel layering or bedding less than one centimeter in thickness, called "fissility" (~splitting).^[1] Mudstones, on the other hand, are similar, though they do not show the tendency to split (fissility).

Shales and mudrocks contain roughly 95 percent of the organic matter in all sedimentary rocks. However, this amounts to less than one percent by mass in an average shale. Black shales form in anoxic (without oxygen) conditions. They contain free carbon and iron sulfides such as pyrite. This produces the black colouration.^[1]

1. ↑ ^{1.0 1.1 1.2} Blatt, Harvey and Robert J. Tracy 1996. Petrology: igneous, sedimentary and metamorphic. 2nd ed, Freeman, 281 - 292 ISBN 0-7167-2438-3