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Department of Mineralogy and Petrology
Faculty of Natural Sciences
Comenius University in Bratislava

Volcanic glass

Volcanic glass is product of rapid quenching of melt. It can either create an individual rock-type or is a significant matrix constituent of volcanic and pyroclastic rocks. Volcanic glass is amorphous, however, it exhibit signs of a polymer-like ordering over short distances. Due to very short cooling, individual atoms did not have time to occupy structural ordering known from minerals.

Volcanic glass is important constituent of special glassy volcanic rocks, however, most frequently it occurs only as matrix in volcanic rocks. Glass-bearing granular rocks exhibit hyaloophitic, intersertal and tholeiitic structures. Trachytic and hyalopilitic structure is diagnostic of aphanitic volcanic rocks. Volcanic rocks with high glass proportion (> 80 vol. %) exhibit glassy (vitritic, hyaline) structure formed predominantly by amorphous volcanic glass, less frequently by microliths (germinal forms of crystals), and/or microphenocrysts. In porphyric rocks, where crystals are cemented by glassy matrix, specific vitrophyric structure is observed. Glass can also be important constituent of pyroclastic rocks, having hyaloclastic (porphyroclasts of minerals and volcanic glass) and vitroclastic (porphyroclasts of glass cemented by glass) structures.

Obsidian is natural acidic volcanic glass with low water content (< 1 wt. %), usually showing black, black-grey and grey colours, glassy lustre, and conchoidal fracture. Known are also red, brown-green, green and rarely also transparent varieties. Obsidian originates by chilling of felsic, highly viscous lava. It is fragile, hard, and disintegrates to fragments with sharp edges. In prehistoric times, it was utilized by humans to manufacturing instruments for daily use.

Pumice is bright and light glass with a small water content and numerous pores. Its density is lesser that that of water, and it therefore floats on liquid water. Its chemical composition most frequently corresponds to rhyolite. However, dacitic, rhyodacitic and exceptionally also basaltic pumices also occur.

Pitchstone is a dull black volcanic glass with typical resinous lustre and irregular to hackly fracture. Pitchstone is formed by rapid cooling of viscous rhyolitic magma and usually occurs at the rims of lava flows. Pitchstone comprises small amount of teeny quartz, feldspars and biotite porphyroclasts and/or their microliths. Water content is higher (4-10 wt. %) in comparison to obsidian.

Perlite is volcanic glass with specific jointing, disintegrating in small pearls, which originates as a consequence of rapid cooling accompanied by violent gas release. It typically forms also by hydration of obsidian. Its composition corresponds to rhyolite. Perlite is bright and has a low density. It still contains appreciable amount of water, which triggers its expansion on heating. For this reason, it is used as insulation construction material.

Less common are volcanic glasses of more basic compositions, corresponding to tachylite, sideromelane, palagonite, and specific morphological forms, such as Péĺé´s tears, and Pélé´s hairs.

Tachylite is fragile volcanic glass with low silica content. Dark colours are due to microliths of Fe-Ti oxides. It has a porous texture and occurs along margins of sills and dykes.

Sideromelane is bright, very frequent form of translucent basaltic glass, originating during submarine basaltic eruptions or along contact of basaltic lava with water. In contrast to tachylite, it does not contain microliths of Fe-Ti oxides.

Palagonite is product of weathering of basaltic volcanic glass taking place in aqueous environment. It can originate also at the contact of basaltic lava with water, which coverts to steam. The evolved steam reacts with small fragments of quenched lava and forms bright coloured palagonitic tuffs. Less frequently it originates also by slow cooling of basaltic magma as yellow-orange crust on the surface of lava flows.

"Pélé´s hairs" are weird fibrous forms of basaltic volcanic glass. They are formed from tiny glass droplets ejected in molten state and twisted in the air to long (> 2 m) and thin (< 0.5 mm) threads – hairs. They have golden to golden-brown colours and preserve the orientation of wind blow during volcanic activity. They are called after Pélé – hawaiian goddess of volcanoes.

"Pélé´s tears" are similar in origin to Pélé´s hairs, when small drops of basaltic melt elongate and coalesce when flying in the wind. They have black colours and occur also on tips of the Pélé´s hairs. Origin of the name is the same as in the previous example.