<p>You might be getting <a href="https://en.wikipedia.org/wiki/Sulfite" rel="nofollow noreferrer">sulfite</a>  (SO<sub>3</sub><sup>2−</sup>) and <a href="https://en.wikipedia.org/wiki/Sulfate" rel="nofollow noreferrer">sulfate</a> (SO<sub>4</sub><sup>2−</sup>) mixed up.</p>
<p><a href="https://en.wikipedia.org/wiki/Iron(II)_sulfate" rel="nofollow noreferrer">Iron(II) sulfate</a> is FeSO<sub>4</sub>·xH<sub>2</sub>O. As you state, <a href="https://en.wikipedia.org/wiki/Melanterite" rel="nofollow noreferrer">melanterite</a> is FeSO<sub>4</sub>·7H<sub>2</sub>O, which is a mineral form of hydrous iron(II) sulfate, an iron analogue of the copper sulfate chalcanthite.</p>
<p>Various sources list the color of <a href="https://www.mindat.org/min-2633.html" rel="nofollow noreferrer">melanterite</a> as being</p>
<blockquote>
<p>Colorless to white or green, also greenish-blue to blue with increased Cu content; colourless to pale green in transmitted light.</p>
</blockquote>
<p><a href="https://i.sstatic.net/CWHj7.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/CWHj7.jpg" alt="Melaterite_1" /></a></p>
<p><a href="https://mineralexpert.org/article/melanterite-iron-sulfate-mineral-overview" rel="nofollow noreferrer">Another</a> states (the emphasis is mine),</p>
<blockquote>
<p><strong>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</strong>.</p>
<p>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.</p>
<p>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 H<sub>2</sub>O), siderotil (5x H<sub>2</sub>O), rozenite (4x H<sub>2</sub>O) and szomolnikite (1x H<sub>2</sub>O).</p>
<p>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 Fe<sup>2+</sup> and Fe<sup>3+</sup> in the structure. These are further oxidized to pure Fe<sup>3+</sup> sulfates and lastly converted to more stable Fe<sup>3+</sup> minerals like goethite, lepidocrocite, or jarosite – depending on acidity of the environment and presence of water/humidity.</p>
</blockquote>
<p><a href="https://i.sstatic.net/0S2oF.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/0S2oF.jpg" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2021-10-04T08:51:18.693Z
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).
David Sterlinsky
CC BY-SA 4.0
2021-10-04T03:52:35.170Z
2021-10-04T03:52:35.170Z
Fred
<p>I've read that melanterite (FeSO<sub>4</sub>*7H<sub>2</sub>O) is colorless or white and gets to be green to blue with increase in Cu<sup>2+</sup> 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 ?</p>

<mineralogy><minerals>
Why is Melanterite white or colorless?
David Sterlinsky
# Why is Melanterite white or colorless?

(Asked by: username_0 on 2021-10-03)

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 ?

**Tags:** mineralogy, minerals


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## 1 answers

### Answer by username_1 on 2021-10-04. Score: 3 | Accepted answer

You might be getting [sulfite](https://en.wikipedia.org/wiki/Sulfite) (SO32−) and [sulfate](https://en.wikipedia.org/wiki/Sulfate) (SO42−) mixed up.

[Iron(II) sulfate](https://en.wikipedia.org/wiki/Iron(II)_sulfate) is FeSO4·xH2O. As you state, [melanterite](https://en.wikipedia.org/wiki/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](https://www.mindat.org/min-2633.html) 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](https://i.sstatic.net/CWHj7.jpg)](https://i.sstatic.net/CWHj7.jpg)

[Another](https://mineralexpert.org/article/melanterite-iron-sulfate-mineral-overview) 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.

[![enter image description here](https://i.sstatic.net/0S2oF.jpg)](https://i.sstatic.net/0S2oF.jpg)

> **Comments:**
> **username_0**: 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).  

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