Although sometimes considered as clay mineral, glauconite is more accurately a member of the mica group, with the composition (K,Ca,Na)2(Fe3+,Al,Mg,Fe2+)4[(Si,Al)4O10]2(OH)4; sp. gr. 2.4–3.0; hardness 2; monoclinic; olive green, yellowish, or blackish green; dull lustre; granular; occurs in marine sediments as aggregates up to 1 mm in diameter. It is being formed on many modern continental shelves, at depths from a few tens to hundreds of metres, where the sedimentation rate is low and decaying organic matter is present in a generally oxidizing environment. In many sandstones glauconite can impart a green colour when abundant, as in the Cretaceous Greensands of Britain and the eastern USA.
Subjects: Earth Sciences and Geography.