Epidote often represents alteration of the dark mafic minerals in igneous rocks such as olivine, pyroxene, amphiboles and plagioclase. It indicates a level of metamorphism between greenschist and amphibolite, particularly at low temperatures. Epidote thus is well known in subducted seafloor rocks. Epidote also occurs in metamorphosed limestones.
The molecular structure of epidote is hard to describe: chains of aluminum oxide octahedra alternate with hydroxylated versions, and silica units both single (SiO4) and doubled (Si2O7) bind the chains together. Rare earths may replace the calcium, and the ferric iron (Fe3+) may be replaced by trivalent (Al, Mn3+, V3+) or divalent (Fe2+ or Mg) species.