Muscovite with pleochroic halos? 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



In high resolution here: Biotite with pleochroic halos (30 µm thin section, XPL).

XPL



In high resolution here: Biotite with pleochroic halos (30 µm thin section, XPL).
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.