Greensand is a mineral that contains an iron-potassium silicate known as glauconite.
Green mud differs to a greater extent from the blue mud, and owes its characteristic nature and colour to the presence of glauconite, which is formed inside the cases of foraminifera, the spines of echini and the spicules of sponges in a manner not yet understood.
Secondary products, such as glauconite, phosphatic concretions and manganese nodules, occur though less frequently than in the hemipelagic sediments.
Minerals, like glauconite, which contain ferrous silicate, may in like manner yield limonite, on weathering.
Dark " green-sands," very rich in glauconite, are followed by yellow sandstones with some flint.
In a few places a red colour prevails, the iron being mostly oxidized; elsewhere the muds are green owing to abundant glauconite.
The most distinctive feature of the Cretaceous of the Atlantic coastal plain is its large content of greensand marl (glauconite).