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A glass, as I understand it, is a material whose solid phase is amorphous and which undergoes a glass transition when heated.

My train of thought - and therefore my questions - began with obsidian. My understanding is that obsidian is formed from material that would normally form crystalline rock, but since it's exposed to air during an eruption, it cools too rapidly to crystallize and therefore solidifies as glass. This led me to wonder: how large can a lava stream be and still produce obsidian? In particular, in season 7 of Game of Thrones, the island of Dragonstone is revealed to contain a veritable mountain of obsidian - is such a thing possible in the real world? I would have thought that, above a certain volume, the center of the mass would be well-enough insulated so that crystallization could occur, spoiling the obsidian. What's the largest mass of obsidian found in nature?

Thinking about too-rapid cooling led me to another question: can water form glass? Is it possible to blast-freeze water so quickly that ice crystals do not form? If so, what would be the properties of such a glass?

Finally, I wonder about regular, garden-variety silicate glass (as found in windows and bottles.) Can it be cooled slowly enough so that crystals do form? And if so, what are its properties? (Have I essentially re-invented quartz?)

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    $\begingroup$ WRT existing in the real world, there's Glass Mountain in Northern California: 101things.com/shasta/medicine-lake-glass-mountain There's also another "Glass Mountain" near Mono Lake, in eastern central California, but I think there are only scattered outcrops of obsidian there. $\endgroup$
    – jamesqf
    Commented Sep 9, 2017 at 18:47
  • $\begingroup$ I hadn't known before about Glass Mountain, but it looks like the obsidian was formed in a flow over one of the slopes of a previously-existing mountain of (crystalline, presumably) lava rock. So it's a large obsidian formation, but not quite of the same sort as the Game of Thrones formation (which is buried under the island - if Martin or Benioff and Weiss gave any thought to geology at all, did they imagine the lump of obsidian forming first, then being buried? 'Cause I can't see how it would be possible the other way around. And yes - I know it's fantasy, but hey.) $\endgroup$
    – MT_Head
    Commented Sep 9, 2017 at 21:14
  • $\begingroup$ I know nothing about "Game of Thrones", other than what I've gathered from passing references. But I would think that if you can have one large lava flow solidifying to obsidian, then at least in theory you could have multiple eruptions of the same sort, perhaps enough to build a mountain. $\endgroup$
    – jamesqf
    Commented Sep 10, 2017 at 5:01

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Q1: A volcano consists of many lava flows that occur over a long period of time. In general, only very few of these lava flows have the right composition and the right cooling history that are required to form obsidian.

This is why only parts of a volcano consist of obsidian, and by volume it's a generally a very small amount.

Q2: And yes there is amorphous ice. It is known since the 1980s. These days it is being used in laboratory environments, e.g. for electron cryomicroscopy. This is a high-tech-method for observing biochemical reactions and cellular processes in situ. The microorganisms need to be alive and in the middle of a reaction when shock-freezed. Researchers want to image atomic structures of the biomolecules, not from ubiquitous crystalline water so it better be amourphous.

Q3: Glass from windows and bottles contains more Na and K than most Granitic rocks. Na- and K-rich ingredients are being added as fluxing agent (limestone, soda), in the glass factory, decreasing the melting point of the silicates and therefore lowering the production costs.

I think a melt of bottle glass, when crystallizing slowly would initially form the minerals Nepheline and Leucite, but late in the solidifcation process also Alkali Feldspar, and some Quarz, among other minerals.

If you cooled it so slowly that no glass phase forms, you still might get zoned crystals, having with a different composition from core to rim. That depends on the absence/presence of impurities and local chemical (dis-) equilibria.

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  • $\begingroup$ Eutectic solutions reduce energy use and material demands, but there is another benefit. They do not change the viscosity at a given temperature very much (I don't think so at least). That means the viscosity at the (much lower) melting point is higher, which makes avoiding crystallization easier. $\endgroup$
    – Kevin Kostlan
    Commented Apr 11, 2022 at 18:52
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In particular, in season 7 of Game of Thrones, the island of Dragonstone is revealed to contain a veritable mountain of obsidian - is such a thing possible in the real world? I would have thought that, above a certain volume, the center of the mass would be well-enough insulated so that crystallization could occur, spoiling the obsidian.

Sure, an island of obsidian could form as result of repeated felsic eruptions depositing layer upon layer of obsidian. But, it'd be a relatively recent eruption as obsidian is mineralogically unstable and devitrifies into crystalline chrystobalite, as it slowly, ever so slowly becomes quartz.

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    $\begingroup$ How slowly? Is obsidian really "mineralogically unstable" near the surface, or is this only true at elevated temperatures and pressures after subduction? Could you add a supporting link or reference for this? Thanks! $\endgroup$
    – uhoh
    Commented Sep 10, 2017 at 3:23
  • $\begingroup$ No subduction required...(although stability in a museum display case is likely pretty good). Given the composition of obsidian, remember- it's not "glass" but technically a highly felsic polymer containing all of the elements that make it pretty close to silicic rhyolite in composition, it takes is some near-surface hydrothermal conditions to initiate the re-ordering of SIO4 tetrathedral framework to a more stable phase. Of course the question is how and why it even forms in an unstable state to begin with. Look up: "devitrification of geologic glasses" to see how I mangled my facts $\endgroup$
    – Knob Scratcher
    Commented Sep 11, 2017 at 1:01
  • $\begingroup$ That's fascinating. Wow, devitrification is a thing! I'll go off now and read all about it, thank you! $\endgroup$
    – uhoh
    Commented Sep 11, 2017 at 5:16

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