glauconite

[(K,Na,Ca) 1.2-2.0(Fe+3,Al,Fe+2,Mg)4(Si7-7.6Al1-0.4O20)(OH)47nH20] A green silicate mineral found in sedimentary rocks and formed on continental shelves characterized by slow sedimentation and organic matter, such as fecal pellets, present in an oxidizing environment. In sufficient quantity, it can form a sandy, green deposit such as the Cretaceous greensands of the US and UK.