Volcanic glass

As volcanic glass or rocks glass is a volcanic rock ( or a part of a rock ) is called, which does not consist of minerals, but is present in the amorphous state than glass. This is due to very rapid cooling or quenching of lava or magma of a, so that no crystallization could take place. In volcanic glass is not a separate type of rock, but to a certain rock fabric. The best known example of Mineral glass is obsidian.

Formation

A rock can completely ( hyaline ) or only partially ( hypo- crystalline) consist of rock glass ( as opposed to a glass-free, holokristallinen rock). There are all transitions between hyaline and holokristallinen rocks. In a hyaline rocks occur only sporadically in small crystals which can form a floating structure under certain circumstances. A porphyritic rock in which single larger mineral grains occur as inclusions in a glassy matrix, Vitrophyr is called.

The necessary for the formation of rocks glass rapid cooling is in nature most often achieved by the contact of lava with water or glacial ice. In an explosive volcanic eruption Lavapartikel be smaller ( tephra ) quenched on their transport through the air. Typical is the occurrence of volcanic glasses at the contact between hot magma and water. As a pillow or pillow lava they are formed at mid-ocean ridges and other submarine volcanoes. Also at subglacial volcanoes (eg panel volcanoes ) occur on rock glasses. In the contact area of shallow, small-scale intrusive bodies ( dykes or sills ) to the cold country rock, a glassy quench zone may develop while the interior is present more or less crystallized.

Structure

Volcanic glasses formed from melts of different composition. The chemistry of silica- rich and therefore very viscous ( viscous ) glass-forming molten rock varies from rhyolithisch to phonolithisch about trachytisch and andesitic. SiO2 - poor rocks glasses are basaltic composition. A rock glass is essentially a super-cooled melt, which is in a metastable state. The molecules are in the glass further in a disordered amorphous state, as in the original melt rather than in the ordered crystal structure of a solid. Due to its metastability, the glass is transformed over geological time in a thermodynamically stable crystal structure at ( recrystallization or devitrification ). The crystallization of the glass comes from crystallization nuclei ( small crystals or impurities ) to form what is radial spherulites.

Examples

• Obsidian is a prominent example of a volcanic glass that was used by Stone Age cultures because of its fracture properties preferred for making tools and weapons. Depending on the chemistry we speak of rhyolithischem, phonolithischem andesitic etc. obsidian.
• Pechstein is a Paläovulkanit, which is partially devitrified, containing up to 10 % H2O. Pechstein is clear from geologically ancient obsidian.
• Perlite also arises from the alteration of obsidian and is made to centimeter-sized hydrated glass beads with concentric- peel-like structure.
• Tachylit is a basaltic rock glass, which is found, inter alia, as a deterrent crusts around pillow lavas.
• Palagonite is another glass of basaltic composition, which is produced by absorption of water from other primary rocks glasses ( Palagonitisierung ). In Iceland palagonite is widespread.
• Hyaloklastit refers to a glass containing pyroclastic rock, which is formed by fragmentation on contact to water or glacial ice.

• Pumice is a very porous or frothy glass, which is produced from gas- rich magma during explosive volcanic eruptions.
• Limburgit is now uncommon name for a hypo- crystalline, porphyritic rock basanitischer composition.
• Reticulit is an extremely porous and lightweight, basaltic volcanic glass

Swell

• Richard V. Dietrich, Brian J. Skinner: The rocks and their minerals. Ott Publishers, Thun 1979, ISBN 0-471-02934-3