#wheathering

Green Vitriol is an older name for iron (II) sulfate. A blue-green crystal substance. Is there anything special about it?

Well, first, let’s remember the iron (II) cation is colorless. And indeed, anhydrous iron (II) sulfate is a white powder. Dissolved in water it forms a pale green   [Fe(H2O)6]3+ cation where 6 water molecules surround the iron “core”. But also in it’s solid state iron (II) sulfate binds water molecules. The heptahydrate or FeSO4·7H2O  is the most common example. And it’s green because it also contains six water molecules around the iron core. Same as in water solution. In fact, it’s a kind of “solid solution” of.

The second fact on green vitriol is that it’s susceptible to weathering. If you look at this green powder, you can easily notice brownish impurities. Iron (II) sulfate reacts with oxygene and water from the air:

4FeSO4 + O2 + 2H2O → 4Fe(OH)SO4

In the structure of iron (II) sulfate heptahydrate six water molecules forms a stronger bond with the “iron core” than the seventh one. It’s placed outside the crystal lattice and separates more easily. Thus, the structure can be rather described like  [Fe(H2O)6] SO4·H2O. Another substance, the Mohr’s salt or ammonium iron (II) sulfate or (NH4)2Fe(SO4)2·6H2O is more resistant to weathering or oxidation as it only contains 6 water molecules and all of them are connected to the iron. It’s also easier to work with.

Other hydrated forms are also known. FeSO4·4H2O can be crystallized from the hot solutions within the temperature range between 56C and 64C and  FeSO4·H2O above 64C. 

And according to WIkipedia the following hydration states of iron (II) suflate can be found 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)