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I've read that melanterite (FeSO4*7H2O) is colorless or white and gets to be green to blue with increase in Cu2+ impurities. But from my chemistry experience I know that iron sulfate is green when in solution and when hydrated. So why the mineral is white ? What is different here ?

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You might be getting sulfite (SO32−) and sulfate (SO42−) mixed up.

Iron(II) sulfate is FeSO4·xH2O. As you state, melanterite is FeSO4·7H2O, which is a mineral form of hydrous iron(II) sulfate, an iron analogue of the copper sulfate chalcanthite.

Various sources list the color of melanterite as being

Colorless to white or green, also greenish-blue to blue with increased Cu content; colourless to pale green in transmitted light.

Melaterite_1

Another states (the emphasis is mine),

Fresh melanterite is always green. It tends towards blue-green hues with higher Cu amount in the structure. And it turns pale green to almost white when dehydrated.

Melanterite is highly unstable material. It makes almost no sense to collect it as it requires very specific humidity and acidity. When removed from e.g. underground mines to the surface environment, it gets dehydrated and/or oxidized within hours or maximum few days.

Melanterite is very easily dehydrated. It first turns into pale green color and then into white powdery material when almost completely dry. This is often mixture of various less hydrated iron sulfates like ferrohexahydrite (6x H2O), siderotil (5x H2O), rozenite (4x H2O) and szomolnikite (1x H2O).

Divalent iron in melanterite is also very easily oxidized, which again results into decomposition of melanterite. There are various sulfates with varied amount of water and different ratio of Fe2+ and Fe3+ in the structure. These are further oxidized to pure Fe3+ sulfates and lastly converted to more stable Fe3+ minerals like goethite, lepidocrocite, or jarosite – depending on acidity of the environment and presence of water/humidity.

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  • $\begingroup$ Yeah, that makes a lot of sense to think it gets dehydrated, because indeed anhydrous iron(II) sulfate is white. Thank you very much, also thank you for the sulfite part, it was a typo that I haven't noticed (now I've edited it out). $\endgroup$ Oct 4, 2021 at 8:51

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