Hydrates[edit]
Iron(II) sulfate can be found in various states of hydration, and several of these forms exist in nature.
FeSO4·H2O (mineral: Szomolnokite,[7] relatively rare)
FeSO4·4H2O (mineral: Rozenite,[8] white, relatively common, may be dehydratation product of melanterite)
FeSO4·5H2O (mineral: Siderotil,[1] relatively rare)
FeSO4·6H2O (mineral: Ferrohexahydrite,[2] relatively rare)
FeSO4·7H2O (mineral: Melanterite,[9] blue-green, relatively common)
Iron(II) sulfate anhydrous.
The tetrahydrate is stabilized when the temperature of aqueous solutions reaches 56.6 °C (133.9 °F). Then at 64.8 °C (148.6 °F) they form both tetrahydrate and monohydrate.[4]
All mentioned mineral forms are connected with oxidation zones of Fe-bearing ore beds (pyrite, marcasite, chalcopyrite, etc.) and related environments (like coal fire sites). Many undergo rapid dehydration and sometimes oxidation.