Volcanic glass

Volcanic glass is the amorphous (uncrystallized) product of rapidly cooling magma. Like all types of glass, it is a state of matter intermediate between the close-packed, highly ordered array of a crystal and the highly disordered array of gas. Natural volcanic glasses derive from silicate melts and may vary in composition over the range of common igneous rocks (from 40 to 77 Wt% of SiO₂), excluding ultramafic types. Most rocks that are largely glass are rhyolite, however, interstitial basaltic glass is common in basalt flow. The field classification of glasses is based largely on structure, because chemical composition is only inferred, as follow:

• Obsidian: is usually a black, massive glass with a shiny, vitreous luster and conchoidal fracture. Is a rhyolitic glass with less than 0.5 % of water.
• Perlite: is a gray or pale brown glass, very brittle and highly fractured, with spherical cracks. Perlite glass probably result from hydration of obsidian.
• Pumice: is a highly vesiculated, cellular glass froth.
• Pitchstone: is a massive dark glass that displays a resinous or pitchy luster and conchoidal fracture. It can be rhyolitic or trachyitic and owes its pitchy luster to either abundant crystallites.
• Vitrophire: is a porphyric glass, usually obsidian or Pitchstone, containing phenocrysts of sanidine, plagioclase, biotite and quartz.
• Tachylite or Sideromelane: is a dark basaltic glass common in the groundmass of basalts or as thin shells on pillow lavas. It ranges in silica content between 40 to 55 Wt%.

The most common form of alteration (hydrothermal or weathering) in any natural glass is late-stage devetrification, whereby the unstable amorphous state of an acidic glass crystallizes to a microcrystalline uniform aggregate of quartz and feldspar. Essentially all glass older than Mesozoic has been devitrified. An early devetrification state may produce spherulites in the supercooled glass. Basaltic glass devitrified to a yellow or orange isotropic mineraloid (Palagonite)

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Photo
Volcanic glass with flow structures in a Ignimbrite. PPL image, 2x (Field of view = 7mm)	Volcanic glass with flow structures in a Ignimbrite. PPL image, 2x (Field of view = 7mm)	Volcanic glass with flow structures in a Ignimbrite. PPL image, 2x (Field of view = 7mm)
Volcanic glass with flow structures in a Ignimbrite. PPL image, 2x (Field of view = 7mm)	Volcanic glass with flow structures in a Ignimbrite. PPL image, 2x (Field of view = 7mm)	Rhyolite with glassy groundmass. PPL image, 2x (Field of view = 7mm)
Rhyolite with glassy groundmass. PPL image, 2x (Field of view = 7mm)	Rhyolite with glassy groundmass. PPL image, 10x (Field of view = 2mm)	Rhyolite with glassy groundmass. PPL image, 10x (Field of view = 2mm)
Rhyolite with glassy groundmass. PPL image, 10x (Field of view = 2mm)	Rhyolite with glassy groundmass. PPL image, 10x (Field of view = 2mm)	Rhyolite with glassy groundmass. PPL image, 10x (Field of view = 2mm)
Glass fragments in a Ignimbrite, Bingol (Turkey). PPL image, 10x (Field of view = 2mm)	Glass fragments in a Ignimbrite, Bingol (Turkey). PPL image, 10x (Field of view = 2mm)	Glass fragments in a Ignimbrite, Bingol (Turkey). PPL image, 10x (Field of view = 2mm)
Glass fragments in a Ignimbrite, Bingol (Turkey). PPL image, 10x (Field of view = 2mm)	Glass fragments in a Ignimbrite, Bingol (Turkey). PPL image, 10x (Field of view = 2mm)	Glass fragments in a Ignimbrite, Bingol (Turkey). PPL image, 10x (Field of view = 2mm)
Glass fragments in a Ignimbrite, Bingol (Turkey). PPL image, 10x (Field of view = 2mm)	Glass fragments in a Ignimbrite, Bingol (Turkey). PPL image, 10x (Field of view = 2mm)	Glass fragments in a Ignimbrite, Bingol (Turkey). PPL image, 10x (Field of view = 2mm)
Dendritic crystals in a pitchstone, Arran, Scotland. PPL image, 2x (Field of view = 7mm)	Dendritic crystals in a pitchstone, Arran, Scotland. PPL image, 2x (Field of view = 7mm)	Dendritic crystals in a pitchstone, Arran, Scotland. PPL image, 2x (Field of view = 7mm)
Dendritic crystals in a pitchstone, Arran, Scotland. PPL image, 2x (Field of view = 7mm)	Dendritic crystals in a pitchstone, Arran, Scotland. PPL image, 2x (Field of view = 7mm)	Dendritic crystals in a pitchstone, Arran, Scotland. PPL image, 2x (Field of view = 7mm)