What are Tektites?
Tektites (from Greek τεκτός tektos, molten) are gravel-size bodies that are composed of black, but also green, brown or gray, natural glass, which is characterized by 1. a fairly homogeneous composition; 2. an extremely low content of water and other volatiles; 3. an abundant lechatelierite; 4. a general lack of microscopic crystals known as microlites and chemical relation to the local bedrock or local sediments; and 5. their distribution within geographically extensive strewnfields. Tektites generally range in size from centimeters to millimeters. Millimeters-size tektites are known as microtektites.
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.