vitrophyre - larger than normal hand/display specimen of an unusual volcanic glass
Sometimes vitrophyre is described as obsidian with phenocrysts. It's not as pure a glass as obsidian, but in this case, the origin is interesting. The source of this vitrophyre was a pyroclastic flow of glowing hot volcanic ash, forming a tuff bed in the Resting Springs Formation. The ash bed was thick enough that it served as an insulating blanket for its center. The ash at the center was so hot that it melted into a glass - vitrophyre - and cooled before many crystals could form. Contrast this with an obsidian flow, where a granitic composition lava cooled so rapidly at the surface that there was no time for atoms to arrange into crystals.
On either side of this vitrophyre, the hot ash flow solidified into a densely welded tuff - vitreous instead of ashy as are most tuffs. The densely welded tuff contains fiamme or "flames" in Italian, pieces of pumice that because of the heat, were squashed into almost flat glassy lense-like structures. Potassium-argon dating gives the age of the tuff and vitrophyre as 9.5 million years.
These vitrophyre specimens are larger than normal for this flow, which is fairly well fractured. The pencil is 5" long, for scale.
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