Tektite

Tektite

Tektites (from Greek τεκτός tektos, molten) are gravel-size bodies that are composed of black, green, brown or gray, natural glass that are formed from terrestrial debris ejected during extraterrestrial impacts. They are 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.

On the basis of morphology and physical characteristics, tektites have traditionally been divided into four groups. The tektites, which have been found on land, have traditionally been subdivided into three groups (1) splash-form (normal) tektites, (2) aerodynamically shaped tektites, and (3) Muong Nong-type (layered) tektites. Splash-form and aerodynamically shaped tektites are only differentiated on the basis of their appearance and some of their physical characteristics. Splash-form tektites are centimeter-sized tektites that are shaped like spheres, ellipsoids, teardrops, dumbbells, and other forms characteristic of isolated molten bodies. They are regarded as having formed from the solidification of rotating liquids, and not atmospheric ablation. Aerodynamically shaped tektites, which are mainly part of the Australasian strewn field, are splash-form tektites (buttons), which display a secondary ring or flange. The secondary ring or flange is argued as having been produced during the high-speed reentry and ablation of a solidified splash-form tektite into the atmosphere. Muong Nong tektites are typically larger, greater than 10 cm in size and 24 kg in weight, irregular, and layered tektites. They have a chunky, blocky appearance, exhibit a layered structure with abundant vesicles, and contain mineral inclusions, such as zircon, baddeleyite, chromite, rutile, corundum, cristobalite and coesite.

Microtektites, the fourth group of tektites, are tektites that are less than 1 mm in size. They exhibit a variety of shapes ranging from spherical to dumbbell, disc, oval, and teardrop. The color of microtektites ranges from colorless and transparent to yellowish and pale brown. They frequently contain bubbles and lechatelierite inclusions. Microtektites are typically found in deep-sea sediments that are of the same age as one of the four known strewn fields. Microtektites, which are part of the Australasian strewn field, have also been found on land within Chinese loess deposits and sediment-filled joints and decimeter-sized weathering pits developed within glacially eroded granite outcrops of the Victoria Land Transantarctic Mountains, Antarctica.