Tektites

MINING GEOLOGY
28 p. : ill. ; 25 cm. 2nd rev. ed. Bibliography: p. 28. #290123
Tektites (from Ancient Greek: τηκτός tēktós, meaning ‘molten’) are gravel-sized bodies composed of black, green, brown or grey natural glass formed from terrestrial debris ejected during meteorite impacts. The term was coined by Austrian geologist Franz Eduard Suess (1867–1941), son of Eduard Suess.[note 1][1] They generally range in size from millimetres to centimetres. Millimetre-scale tektites are known as microtektites.[2][3][4]

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 strewn fields
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.[2][3][4] The difference in water content can be used to distinguish tektites from terrestrial volcanic glasses. When heated to their melting point, terrestrial volcanic glasses turn into a foamy glass because of their content of water and other volatiles. Unlike terrestrial volcanic glass, a tektite produces only a few bubbles at most when heated to its melting point, because of its much lower water and other volatiles content.[5]