Glauconite is a phyllosilicate (mica group) mineral of formula: (K,Na)(Fe3+,Al,Mg)2(Si,Al)4O10(OH)2. It can also be referred to as an iron silicate. It crystallizes with monoclinic geometry. The name is derived from the Greek glaucos (??a????) meaning 'gleaming' or 'silvery', to describe the appearance of the blue-green color, presumably relating to the sheen and blue-green colour of the sea's surface. Its color ranges from olive green, black green to bluish green. It is probably the result of the iron content of the mineral. In the Mohs scale it has hardness of 2. The relative density range is 2.2 - 2.8. It is normally found in dark green rounded nodules of sand size dimension. It can be confused with chlorite or with some clays.
Normally, glauconite is considered diagnostic of continental shelf marine depositional environments with slow rates of accumulation. Typically, it appears in Jurassic/lower Cretaceous deposits of Greensand, so-called after the colouration provided by the glauconite. It can also be found in impure limestones, such as Kentish Rag and in Chalk. It develops as a consequence of diagenetic alteration of sedimentary deposits, changes in the biotite micas, for example, being influenced by the decaying process of the organic matter in animal shells. Glauconite forms under reducing conditions in sediments and such deposits are commonly found in nearshore sandstones, open oceans and the Mediterranean Sea but not in the Black Sea or in fresh-water lakes. It oxidises on contact with air; the resulting deoxygenated air is a hazard to miners.
Glauconite has long been used in Europe as an artistic oil paint, especially in Russian "icon paintings". It is rated as a highly permanent pigment, and often marketed by the names "terre verte" or "green earth".
The wide distribution of these sandy deposits was first made known by naturalists on board H.M.S. Challenger (expedition of 1872-1876.