Glauconite

Glauconite, a greenish mineral of variable composition but essentially a hydrous silicate of aluminium, iron, and potassium, which occurs in grains in various sedimentary rocks, especially in the lower part of the Cretaceous system (q.v.), where it gives their names to the Greensands and the Glauconitic (Chloritic) Marl. The grains are sometimes casts of the shells of Foraminifera, and the mineral is now forming similar casts off the coasts of Georgia and Carolina.