A modulated 1:1 layer silicate based on the serpentine structure, with an approximate ideal composition of Fe2+3Si2O5(OH)4. Mn, Mg, and Al can substitute for Fe. There is an apparent excess of Si and an apparent deficiency in octahedral composition on the basis of 7 oxygen atoms. Earlier literature erroneously described greenalite as an iron serpentine, similar to the structure of lizardite. The 1T polytype (space group P31m) is dominant and the 1M polytype (space group Cc) is often intergrown. Greenalite is an “island” structure where Si-rich tetrahedra of a given layer have apical oxygen atoms coordinate to one octahedral sheet and others to the adjacent sheet (Guggenheim and Eggleton, 1998). The islands are saucer-shaped with some islands inverted, and the islands are domed. Island diameters depend on composition with larger-diameter islands having smaller average octahedral cation sizes (4 tetrahedral-ring diameters in greenalite, 3 rings in the Mn analogue, caryopilite). Island domains are randomly displaced within layers. Greenalite is commonly found in Precambrian iron formations.
Cf., caryopilite