<p>There are several options.</p>

<ol>
<li>During decompression, biotite is no longer stable and the hydrogen volatilises, leaving behind fine-grained intergrowth of oxide and silicate minerals.</li>
<li>Oxidation of the rims during exposure of the lava to the atmosphere. Higher Fe<sup>3+</sup> contents would make the biotite darker.</li>
</ol>

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

CC BY-SA 4.0
2019-09-20T07:15:10.803Z
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?
Muharrem Yavuz
CC BY-SA 4.0
2019-09-20T07:37:52.867Z
@MuharremYavuz that is one possible explanation, yes.
Gimelist
CC BY-SA 4.0
2019-09-20T07:03:34.800Z
2019-09-20T07:03:34.800Z
Gimelist
<p><a href="https://i.sstatic.net/6lkiw.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/6lkiw.jpg" alt="enter image description here"></a>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? </p>

CC BY-SA 4.0
2019-09-19T22:27:29.213Z
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.
g.z.
CC BY-SA 4.0
2019-09-19T23:36:00.697Z
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
Muharrem Yavuz
<petrography>
What do opaque rims on biotite in a volcanic rock mean?
Muharrem Yavuz
# What do opaque rims on biotite in a volcanic rock mean?

(Asked by: username_0 on 2019-09-19)

[![enter image description here](https://i.sstatic.net/6lkiw.jpg)](https://i.sstatic.net/6lkiw.jpg)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?

**Tags:** petrography

> **Comments:**
> **username_1**: 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.  
> **username_0**: 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  

---

## 1 answers

### Answer by username_2 on 2019-09-20. Score: 1 | Accepted answer

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.

> **Comments:**
> **username_0**: 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?  
> **username_2**: @username_0 that is one possible explanation, yes.  

---

