Fougèrite
Fougèrite is a relatively recently described naturally-occurring green rust mineral. It is the archetype of the fougèrite group in the larger hydrotalcite supergroup of naturally-occurring layered double hydroxides. The structure is based on brucite-like layers containing Fe and Fe cations, O and OH anions, with loosely bound [CO3] groups and H2O molecules between the layers. Fougèrite crystallizes in trigonal system. The ideal formula for fougèrite is [Fe4Fe2(OH)12][CO3]·3H2O. Higher degrees of oxidation produce the other members of the fougèrite group, namely trébeurdenite, [Fe2Fe4O2(OH)10][CO3]·3H2O and mössbauerite, [Fe6O4(OH)8][CO3]·3H2O.