What do opaque rims on biotite in a volcanic rock mean? In a volcanic region, i have examined two samples petrographically. One of them has biotites with extensive opacitic rims. The other samples biotites hasn't got this property. They have the same modal mineral contents. What makes the difference?
There are several options.


During decompression, biotite is no longer stable and the hydrogen volatilises, leaving behind fine-grained intergrowth of oxide and silicate minerals.
Oxidation of the rims during exposure of the lava to the atmosphere. Higher Fe3+ contents would make the biotite darker.


It is impossible to tell without a detailed electron microscopy analysis. However, my guess it that it is option 1.