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Tektites (from Greek τηκτός ''tēktós'', "molten") are gravel-size bodies composed of black, green, brown or gray natural glass formed from terrestrial debris ejected during extraterrestrial, meteorite impacts. They generally range in size from millimeters to centimeters. Millimeters-size tektites are known as microtektites.〔French, B. M. (1998) ''Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures.'' LPI Contribution No. 954. Lunar and Planetary Institute, Houston, Texas. 120 pp.〕〔McCall, G. J. H. (2001) ''Tektites in the Geological Record: Showers of Glass from the Sky.'' The Geological Society Publishing House, Bath, United Kingdom. 256 pp. ISBN 1-86239-085-1〕〔Montanari, A., and C. Koeberl (2000) ''Impact Stratigraphy. The Italian Record.'' Lecture Notes in Earth Sciences Series no. 93. Springer-Verlag, New York, New York. 364 pp. ISBN 3540663681〕 Tektites are characterized by: # a fairly homogeneous composition; # an extremely low content of water and other volatiles; # an abundance of lechatelierite; # a general lack of microscopic crystals known as microlites and chemical relation to the local bedrock or local sediments; # their distribution within geographically extensive strewnfields ==Characteristics== Although tektites are superficially similar to some terrestrial volcanic glasses (obsidians), they have unusual distinctive physical characteristics that distinguish them from such glasses. First, they are completely glassy and lack any microlites or phenocrysts, unlike terrestrial volcanic glasses. Second, although high in silica (>65 wt%), the bulk chemical and isotopic composition of tektites is closer to those of shales and similar sedimentary rocks and quite different from the bulk chemical and isotopic composition of terrestrial volcanic glasses. Third, tektites contain virtually no water (<0.02 wt%), unlike terrestrial volcanic glasses. Fourth, the flow-banding within tektites often contains particles and bands of lechatelierite, which are not found in terrestrial volcanic glasses. Finally, a few tektites contain partly melted inclusions of shocked and unshocked mineral grains, i.e. quartz, apatite, and zircon, as well as coesite.〔〔〔 The difference in water content can be used to distinguish tektites from terrestrial volcanic glasses. When heated to their melting point, terrestrial volcanic glasses will turn into a foamy glass because of their content of water and other volatiles. Unlike terrestrial volcanic glass, a tektite will produce only a few bubbles at most when heated to its melting point, because of its much lower water and other volatiles content.〔O'Keefe, J. A., (1978) ''Tektites and Their Origin.'' Developments in petrology vol. 4. Elsevier Scientific Publishing Company, New York, New York. 254 pp. ISBN 9780444413505〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「tektite」の詳細全文を読む スポンサード リンク
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