Volcanic glass

A sand grain of volcanic glass under the petrographic microscope. Its amorphous nature makes it disappear in cross-polarized light (bottom frame). The scale box is in millimeters.

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 closely packed, highly ordered array of a crystal and the highly disordered array of liquid.^[1] Volcanic glass may refer to the interstitial material, or matrix, in an aphanitic (fine-grained) volcanic rock, or to any of several types of vitreous igneous rocks.

Most commonly, it refers to obsidian, a rhyolitic glass with high silica (SiO₂) content.^[2]

Other types of volcanic glass include the following:

• Pumice, which is considered a glass because it has no crystal structure.
• Apache tears, a kind of nodular obsidian.
• Tachylite (also spelled tachylyte), a basaltic glass with relatively low silica content.
• Sideromelane, a less common form of tachylyte.
• Palagonite, a basaltic glass with relatively low silica content.
• Hyaloclastite, a hydrated tuff-like breccia of sideromelane and palagonite.
• Pele's hair, threads or fibers of volcanic glass, usually basaltic.
• Pele's tears, tear-like drops of volcanic glass, usually basaltic.
• Limu o Pele (Pele's seaweed), thin sheets and flakes of brownish-green to near-clear volcanic glass, usually basaltic.

References[]

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