What is "alum shale"

Alum schist \Al"um schist"\, Alum shale \Al"um shale"\, (Min.) A variety of shale or clay slate, containing iron pyrites, the decomposition of which leads to the formation of alum, which often effloresces on the rock.

Alum shale (also known as alum schist and alum slate) is a variety of black shale. It is shale or clay slate containing pyrite. Decomposition of pyrite by weathering forms sulfuric acid, which acts on potash and alumina constituents to form alum, which often occurs as efflorescences on the rock outcrop.

As alum shale contains kerogen originated from algae, it is also classified as marinite-type oil shale. At the same time it is rich in aromatic hydrocarbon attributed to post-depositional irradiation damage induced by uranium concentration in the shale. Alum shale also contains enhanced levels of radium as a result of uranium decay. Between 1950 and 1989, Sweden used alum shale for the uranium production.