Iron(II) Sulfate - Hydrates

Hydrates

Iron(II) sulfate can be found in various states of hydration, and several of these forms exist in nature.

• FeSO₄·H₂O (mineral: szomolnokite, relatively rare)
• FeSO₄·4H₂O (mineral: rozenite, white, relatively common, may be dehydratation product of melanterite)
• FeSO₄·5H₂O (mineral: siderotil, relatively rare)
• FeSO₄·6H₂O (mineral: ferrohexahydrite, relatively rare)
• FeSO₄·7H₂O (mineral: melanterite, blue-green, relatively common)

At 90 °C, the heptahydrate loses water to form the colorless monohydrate. In its anhydrous, crystalline state, its standard enthalpy of formation is Δ_fH°_solid = -928.4 kJ·mol−1 and its standard molar entropy is S°_solid = 107.5 J·K−1·mol−1. 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.