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Encyclopedia of geology, five volume set, volume 1 5 (encyclopedia of geology series) ( PDFDrive ) 1449

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IGNEOUS ROCKS/Obsidian 267 Bickle MJ, Ford CE, and Nisbet EG (1977) The petro genesis of peridotitic komatiites; evidence from high pressure melting experiments Earth Planetary Sci entific Letters 37: 97 106 Echeverria LM (1980) Tertiary or Mesozoic komatiites from Gorgona Island, Colombia; field relations and geochem istry Contributions to Mineral Petrology 73: 253 266 Faure F, Arndt N, and Libourel G (2002) Crystallisation of plate spinifex texture at 1atm pressure in a thermal gradient Geochim Cosmochim Acta v Goldschmidt Conference Abstracts, p A225 Green DH (1974) Genesis of Archaean peridotitic magmas and constraints on Archaean geothermal gradients and tectonics Geology 3: 15 18 Herzberg C (1999) Phase equilibrium constraints on the formation of cratonic mantle In: Fei Y, Bertka CM, and Mysen BO (eds.) Mantle Petrology: Field Obser vations and High Pressure Experimentation, pp 13 46 Houston: The Geochemical Society Jahn BM, Gruau G, and Glickson AY (1982) Komatiites of the Onverwacht Group, South Africa: REE chemistry, Sm Nd age and mantle evolution: Contributions to Min eral Petrology 80: 25 40 Le Maitre RW, Bateman P, Dudek A, et al (1989) A Classification of Igneous Rocks and Glossary of Terms Oxford: Blackwell Lesher CM, Arndt NT, and Groves DI (1984) Genesis of komatiite associated nickel sulphide deposits at Kambalda, Western Australia: A distal volcanic model In: Buchanan DL and Jones MJ (eds.) Sulphide Deposits in Mafic and Ultramafic Rocks, pp 70 80 London: Institution of Mining and Metallurgy Nesbitt RW, Jahn BM, and Purvis AC (1982) Komatiites: an early Precambrian phenomenon Journal of Volcanic Geothermal Research 14: 31 45 Nisbet EG (1982) The tectonic setting and petrogenesis of komatiites In: Arndt NT and Nisbet EG (eds.) Komatiites, pp 501 520 London: George Allen and Unwin Nisbet EG, Cheadle MJ, Arndt NT, and Bickle MJ (1993) Constraining the potential temperature of the Archaean mantle: a review of the evidence from komatiites Lithos 30: 291 307 Parman S, Dann J, Grove TL, and de Wit MJ (1997) Em placement conditions of komatiite magmas from the 3.49 Ga Komati Formation, Barberton Greenstone Belt, South Africa Earth Planetary Scientific Letters 150: 303 323 Sun SS and Nesbitt RW (1978) Petrogenesis of Archean ultrabasic and basic volcanics: evidence from rare earth elements Contributions to Mineral Petrology 65: 301 325 Obsidian G J H McCall, Cirencester, Gloucester, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction Obsidian is an extremely siliceous volcanic rock, found in lava flows and volcanic plugs, domes, and necks It is closely related to rhyolite but has solidified from an extremely viscous magma as a glass with only minute crystallites within it Like granite, it has a composition close to the eutectic, which explains the lack of crystallization (i.e., the glass solidified before crystals could form) Obsidian is of rare occurrence in volcanic suites, in which case it is mostly in calcalkaline island-arc-type suites: this rarity is probably because of the fact that obsidian is the final residual product of differentiated magmatic suites, and the fact that the stickiness of the magma restricts it to protrusions such as domes and flows of small extent, which are squeezed out with difficulty It has immense archaeological significance, having been used widely by ancient people for ornaments, arrowheads, knives, and scrapers The Romans and Greeks quarried obsidian extensively for gemstones and the Aztecs mined it in the fourteenth through sixteenth centuries, presumably for ornaments It is still listed as semiprecious gemstone, and the famous jeweller Peter Carl Faberge´ (1846–1920) used it to create animal-shaped ornaments Colour (black, brown, green, yellow, and red), translucency and transparency, reflectance, relative hardness, and sharp edges when fractured are all qualities that have made obsidian desirable through the ages Historical The term ‘obsidian’ is a very ancient word for natural glasses In the first century ad, Pliny the Younger wrote: ‘‘Among the various kinds of glass we may also reckon obsidian, a substance very similar to the stone which Obsidius discovered in Ethiopia The stone is a very dark colour and somewhat transparent, but it is dull to the sight, and reflects, when attached as mirror to walls, the shadow of the object rather than the image’’ Thus the origin of the term clearly goes back to the Romans, and the first record was in Ethiopia That the name of the rock comes from that of a person is, however, doubtful John Hill in 1740 wrote, based on Theophrastus, a Greek philosopher who wrote extensively on plants, stones, and climatic

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