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Devitrification is the crystallization of volcanic glass to crystals. Obsidian groundmass -black in color- turns to white with devitrification.

I want to learn why devitrification takes place.

Attached photo, exhibits an example of devitrification, which i had taken in the field. enter image description here

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  • $\begingroup$ Are you sure it's an ignimbrite? I've never heard of obsidian layers within an ignimbrite deposit. Generally obsidian is associated with effusive volcanism (rhyolite lava flows). In such flows, there are often the two facies (obsidian and crystalline rhyolite) coexisting, see the model of Fink (1983) doi.org/10.1130/0016-7606(1983)94%3C362:SAEOAR%3E2.0.CO;2 (sometimes at a small scale, see flow banding). Such flows are like this from the start, i.e., the rhyolite part is non-glassy from the beginning. But it is true that the obsidian part will tend to devitrify over long timescales. $\endgroup$ Commented Feb 22 at 13:31
  • $\begingroup$ There are other facies in Fink (1983). The succession bottom to top is, Basal Breccia, Coarse Pumice, Obsidian, fine pumice, surface breccia. If i see such a succession in the field i will say this is an ignimbrite not a lava flow. In that paper photos are black-white i i couldn't see textures properly. In my field study i saw eutaxitic and vitrophyric textures origining from a pyroclastic density current but i saw a lithofacies inside this flow unit, which has abundant visible crystals which made me to rethink on Fink 1983. I am considering remelting or mixing. $\endgroup$ Commented Feb 28 at 19:49
  • $\begingroup$ Be careful, there are several examples of alleged "rheomorphic ignimbrites" that have later been proven to be extensive, silicic lava flows. The two can actually be hard to distinguish in the field. Here are a few classic papers about this: - doi.org/10.1007/BF00278387 - doi.org/10.1016/0377-0273(92)90113-R - doi.org/10.1007/s004450050120 $\endgroup$ Commented Feb 29 at 9:58
  • $\begingroup$ I found photos from Glass Mountain. The pumice facies there, seem as an extensive rock which all made of pumice. Pumices aren't existing as an pyroclasts inside the rock. So Little glass mountain has no relation with an ignimbrite as i thought previously. I learned that pumice and obsidian beds can be occured by a lava flow. But one must expect autobrecciation from this lava as similar to basaltic aa flows. Here links of photos highway8a.blogspot.com/2015/11/views-from-glass-mountain.html and americansouthwest.net/california/lava_beds/… $\endgroup$ Commented Feb 29 at 11:05
  • $\begingroup$ It's hard to tell from one picture. It looks like a breccia. Many silicic lava flow have some breccia facies that look like this. Here is a picture I took of Pietre Cotte rhyolite flow (Vulcano, Italy): imgur.com/SMdURoW You can see 3 layers, from top to bottom: massive obsidian facies; a layer of big blocks beginning to break/detach from the upper layer (with some nice flow banding); a layer with many small obsidian clasts that we interpret as a basal breccia (not explosive). Could you share the location (coordinates) of your outcrop? I'd love to look at satellite photos of the area. $\endgroup$ Commented Feb 29 at 11:10

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