<p>It's biotite. Not muscovite. That's why you have the pleochroic halos in it.</p>

<p>I don't understand why you are saying that the interference colours are like muscovite. First of all, the lack of cleavage suggests you are looking down (or close to) the x axis, so this means the interference colours you see are going to be very low. Let's say 1st order white or brown. And then, it's going to get the anomalous brown colour of the biotite together with it, which is exactly what you see here. Also, muscovite is not brown in ppl. This one is. Therefore, it's biotite.</p>

CC BY-SA 3.0
2016-12-02T04:28:16.500Z
2016-12-06T06:16:06.913Z
2016-12-06T06:16:06.913Z
Gimelist
<p>Muscovite CAN in fact show some pleochroic halos in some rare cases... when cordierite (for example in granitic rocks) is altered hydrothermally, sometimes it forms larger areas of Muscovite (instead of some super fine-grained alteration product accumulation).
-&gt; then the halos are obvervably from time to time. But not nearly as intensive as in the usual ingneous biotites.</p>
<p>But as the others already said - Muscovite is never brown in PPL and can only be veeeery slightly greenisch (much less then green biotite varieties), when it derives from Cordierite as an alteration product.</p>
<p>I have no real source of this information other than observing and listening at what our profs said in university, when I had to observe some granitic rocks via photomicrographs for a thesis work.</p>
<p>I know it has been many years since this post was created, but as I am searching through endless pages of google to find any informations and/or pictures of good hydrothermal alteration examples, I thought &quot;If I found this 6-feet-under-buried question-posting, than why can't others stumble across it, too (in the future).</p>
<p>Cheers!</p>

CC BY-SA 4.0
2024-12-05T10:31:54.057Z
2024-12-05T10:31:54.057Z
Anonymous
<p>One doesn't see pleochroic haloes in muscovite, I suspect because the crystal lattice of muscovite isn't conducive to developing inclusions of allanite and other radioactive minerals. I suggest you measure the 2V- to distinguish between muscovite and the paler varieties of biotite. Note that the more magnesian-rich varieties of biotite, <em>i.e.</em> phlogopite, can be very pale brown in PPL. </p>

<p>I don't attach much credence to interference colours because the range of colours in muscovite and phlogopite overlap, and can appear very similar. Also, slightly coloured crystals tend to mask the interference colours, so it may be difficult to judge.</p>

<p>One may ask 'why pleochroic haloes in biotite, but not in muscovite?', as in 2-mica schist. I can't prove it, but it is known that uranium adsorbs onto Fe-rich surfaces of the biotite, so my hypothesis is that adsorbed uranium serves as crystallization foci for adjacent allanite crystalls, which are then enclosed as inclusions as the biotite crystal continues to grow. You won't see this in muscovite because of the absence of Fe.  </p>

<p>It is curious that inclusions of magnetite can occur in muscovite, but not radioactive mineral inclusions - at least, not that I have ever seen. I don't know why. Anyone else have any ideas on this?</p>

CC BY-SA 3.0
2016-12-02T08:21:31.230Z
You should post your last paragraph as a new question -- I for one would be interested to read any answers it may attract.
Pont
CC BY-SA 3.0
2016-12-02T03:48:16.960Z
2016-12-02T03:48:16.960Z
Gordon Stanger
<p>Is it possible to find pleochroic halos in muscovite ? I would have said that it must be biotite (also, in PPL it is brown!), but then.. why does it have interference colors so similar to muscovite ? Or is it anything else ? Thanks for help !</p>

<p><strong>PPL</strong></p>

<p><a href="https://i.sstatic.net/QdQlY.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/QdQlY.jpg" alt="enter image description here"></a></p>

<p>In high resolution here: <a href="https://www.flickr.com/photos/violetplanet/30947378580/in/album-72157671897003403/" rel="nofollow noreferrer">Biotite with pleochroic halos (30 µm thin section, XPL)</a>.</p>

<p><strong>XPL</strong></p>

<p><a href="https://i.sstatic.net/jHtfK.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/jHtfK.jpg" alt="enter image description here"></a></p>

<p>In high resolution here: <a href="https://www.flickr.com/photos/violetplanet/31315634545/in/album-72157671897003403/" rel="nofollow noreferrer">Biotite with pleochroic halos (30 µm thin section, XPL)</a>.</p>

<geology><mineralogy><minerals>
Muscovite with pleochroic halos?
Petr Hykš
# Muscovite with pleochroic halos?

(Asked by: username_0 on 2016-12-01)

Is it possible to find pleochroic halos in muscovite ? I would have said that it must be biotite (also, in PPL it is brown!), but then.. why does it have interference colors so similar to muscovite ? Or is it anything else ? Thanks for help !

**PPL**

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

In high resolution here: [Biotite with pleochroic halos (30 µm thin section, XPL)](https://www.flickr.com/photos/violetplanet/30947378580/in/album-72157671897003403/).

**XPL**

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

In high resolution here: [Biotite with pleochroic halos (30 µm thin section, XPL)](https://www.flickr.com/photos/violetplanet/31315634545/in/album-72157671897003403/).

**Tags:** geology, mineralogy, minerals


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## 3 answers

### Answer by username_3 on 2016-12-02. Score: 3 | Accepted answer

One doesn't see pleochroic haloes in muscovite, I suspect because the crystal lattice of muscovite isn't conducive to developing inclusions of allanite and other radioactive minerals. I suggest you measure the 2V- to distinguish between muscovite and the paler varieties of biotite. Note that the more magnesian-rich varieties of biotite, *i.e.* phlogopite, can be very pale brown in PPL.

I don't attach much credence to interference colours because the range of colours in muscovite and phlogopite overlap, and can appear very similar. Also, slightly coloured crystals tend to mask the interference colours, so it may be difficult to judge.

One may ask 'why pleochroic haloes in biotite, but not in muscovite?', as in 2-mica schist. I can't prove it, but it is known that uranium adsorbs onto Fe-rich surfaces of the biotite, so my hypothesis is that adsorbed uranium serves as crystallization foci for adjacent allanite crystalls, which are then enclosed as inclusions as the biotite crystal continues to grow. You won't see this in muscovite because of the absence of Fe.

It is curious that inclusions of magnetite can occur in muscovite, but not radioactive mineral inclusions - at least, not that I have ever seen. I don't know why. Anyone else have any ideas on this?

> **Comments:**
> **username_4**: You should post your last paragraph as a new question -- I for one would be interested to read any answers it may attract.  

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### Answer by username_1 on 2016-12-02. Score: 6

It's biotite. Not muscovite. That's why you have the pleochroic halos in it.

I don't understand why you are saying that the interference colours are like muscovite. First of all, the lack of cleavage suggests you are looking down (or close to) the x axis, so this means the interference colours you see are going to be very low. Let's say 1st order white or brown. And then, it's going to get the anomalous brown colour of the biotite together with it, which is exactly what you see here. Also, muscovite is not brown in ppl. This one is. Therefore, it's biotite.


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### Answer by username_2 on 2024-12-05. Score: 1

Muscovite CAN in fact show some pleochroic halos in some rare cases... when cordierite (for example in granitic rocks) is altered hydrothermally, sometimes it forms larger areas of Muscovite (instead of some super fine-grained alteration product accumulation).
-> then the halos are obvervably from time to time. But not nearly as intensive as in the usual ingneous biotites.

But as the others already said - Muscovite is never brown in PPL and can only be veeeery slightly greenisch (much less then green biotite varieties), when it derives from Cordierite as an alteration product.

I have no real source of this information other than observing and listening at what our profs said in university, when I had to observe some granitic rocks via photomicrographs for a thesis work.

I know it has been many years since this post was created, but as I am searching through endless pages of google to find any informations and/or pictures of good hydrothermal alteration examples, I thought "If I found this 6-feet-under-buried question-posting, than why can't others stumble across it, too (in the future).

Cheers!


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