enter image description hereIn 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?

  • $\begingroup$ It would be helpful if you could provide scale, type of polarization, region the sample is from, and some arrows labeling minerals on the thin section. $\endgroup$
    – g.z.
    Commented Sep 19, 2019 at 22:27
  • $\begingroup$ Were both samples collected from the same site in the volcanic region? Could one sample have undergone a different depressurization profile than the other leading the more hydrated mineral formation around the crystals than the other sample? Could the samples represent somewhat different magma origins based on where in the volcanic region they were collected. $\endgroup$
    – user824
    Commented Sep 19, 2019 at 22:35
  • $\begingroup$ i mapped them as a same unit. But there is a strong possibility that a fault divides them. And other field evidences says they are not the same unit (at least not the same flow). i'm sure that samples which don't have opacitic rims is lava flow, but the other which has opacitic rims has the possibility to be exogenus dome flow. The differences of the thin sections between them are that one has opacitic rims and the opacitic rim earing one has low biotite amount. The remaing characteristics are same. I'm trying to use this difference to make a conclusion in favor to the existence of new unit $\endgroup$ Commented Sep 19, 2019 at 23:36

1 Answer 1


There are several options.

  1. During decompression, biotite is no longer stable and the hydrogen volatilises, leaving behind fine-grained intergrowth of oxide and silicate minerals.
  2. 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.

  • $\begingroup$ Can effusive or explosive behaviour be responsible for the difference regarding devolatilisation? In explosive volcanism pressure gets high and this inhibits devolatilisation but in effusive behavior it can freely devolatilise. What do you think about this explanation? $\endgroup$ Commented Sep 20, 2019 at 7:15
  • $\begingroup$ @MuharremYavuz that is one possible explanation, yes. $\endgroup$
    – Gimelist
    Commented Sep 20, 2019 at 7:37

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.