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| Composition | Variable composition |
| | Optic class & sign | Isotropic |
| | Relief | Moderate-negative to moderate-positive |
| | Refractive Index | 1.46 -1.62
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| n increases from rhyolitic to basaltic glass |
| | Birefringence | May be weakly birefringent |
| | Colour | Colourless, pale grey, pale yellow, pale brown to reddish, or green, dark brown to opaque |
| | Zoning | |
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| Alteration / decomposition | Glass is commonly subject to devitrification: formation of microcrystalline aggregates of feldspar and quartz (or other silica minerals, like tridymite, cristobalite) in rhyolitic glass; spherulites and/or minute isolated crystals (microlites) form immediately after solidification; basaltic glass converts to isotropic “palagonite”. Low-temperature breakdown products of glass include zeolites and clays.
Glass is unstable and hence does not preserve well in rocks older than Cenozoic.
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| Occurence | Ign | Rapidly quenched volcanic rocks |
| | | Met | Buchite (sanidinite facies); shock metamorphism: diaplectic glass; pseudotachylite, impact- or fault-related |
| | | Sed | |
| | | Hyd | |
| | | Other | Meteorites, tektites, fulgurite |
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| Distinctive properties | Isotropic, non-crystalline, occurrence |
| | Additional comments | Types of volcanic glass and rocks consisting of glassy fragments: obsidian, pumice, perlite, pitchstone, ignimbrite, vitrophyre, vitric tuff, tachylyte. Glassy portions may show flow structures and vesicles. |
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