Tektites


A synposis by member D.M. Schneider

Tektites are glassy objects that are thought by most scientists today to be melt products of terrestrial rocks formed by hypervelocity impacts of large, extraterrestrial objects. They superficially resemble obsidian in appearance and chemical composition; however, several things distinguish these objects from obsidian. Primarily, they have a very low water content, a low alkali content, and they always contain lechatelierite (pure silica glass). They also often contain coesite (a highly dense silica polymorph), nickel-iron spherules, and baddeleyite (a zircon oxide mineral produced at very high temperatures during shock metamorphism), which lend evidence to a meteorite impact origin. Relict mineral inclusions often yield information about the tektite parent material.

Tektites are assigned to strewnfields, which are the areas over which chemically and physically related tektites are found. The assignment of a strewnfield is based on the oxide composition of a tektite. Four of the major strewnfields are the Australasian, Ivory Coast, Czechoslovakian, and North American strewnfields. Strewnfields include tektites, which are found on land, and microtektites, which are microscopic tektites that have been found in deep-sea sediments. Sizes range from less than 1 mm for microtektites to chunks 10-20 cm in width, with most being a centimeter or so in size and weighing a few grams (Glass, 1982). Tektites display a wide array of sizes, shapes, and surface features. For example, there are splash forms that include spheres, teardrops, dumbbells, and discs, ablated forms also known as 'buttons', and chunks known as Muong Nong types that display a layered structure and are found primarily in Southeast Asia.

Mankind's association with tektites goes back to prehistoric man, who used them as implements and ornaments (King, 1977; Glass, 1982). There are tools made of tektites that date back to c. 4,000-6,000 B.C., and after the iron age (500 B.C.) tektites were worn as good luck charms (Bagnall, 1991). The first written reference to tektites appeared c. A.D. 950, when Liu Sun in China named them Lei-gong-mo, which means 'Inkstone of the Thundergod' (Barnes, 1969; Bagnall, 1991). The first reference in scientific literature appeared in 1788, when Mayer described them as a type of terrestrial volcanic glass. In 1900, F.E. Suess coined the term 'tektite' from the Greek word tektos, meaning 'molten'. He was convinced that tektites were of an extraterrestrial origin, and he believed the shapes to be caused by sculpturing due to high velocity air flow. He believed them to be glass meteorites, and because his work was highly read, people began referring to tektites as such; as a result of this, many universities and museums housed tektites with meteorites for many years. This idea was later rejected when no meteorites were found with compositions similar to that of tektites, and when no evidence of cosmic ray exposure was found in tektites. The lack of cosmic ray exposure also led to the idea that tektites could not have evolved outside of an Earth-Moon system, since it indicated that the tektites' time in space had to be less than 900-90,000 years, which is not long enough for anything outside an Earth-Moon system to travel to Earth. (King, 1977; Glass, 1982). In 1917, the meteoriticist F. Berwerth discovered that tektites were similar chemically to certain sedimentary rocks, the first hint of terrestrial origins (King, 1977). The return of lunar materials from the Apollo missions in the late 60s provided evidence that tektites are compositionally unrelated to lunar material, which convinced the majority of scientists that tektites are terrestrial in origin.

Photos: The top photo is of a tektite, probably from Indochina. It is described in a paper by D.A. Kring et al. in the January 1995 issue of Meteoritics (Volume 30, pages 110-112). The bottom photo is of a microtektite collected from the Indian Ocean, showing the 'button' morphology. This sample is described by B.P. Glass et al. in the May 1996 issue of Meteoritics & Planetary Science (Volume 31, pages 365-369).

References
Bagnall, P. M. (1991) Tektites. In The Meteorite & Tektite Collector's Handbook, pp. 113-125. Willmann-Bell, Inc., Richmond, VA.
Barnes, V. E. (1969) Petrology of Moldavites. Geochimica et Cosmochimica Acta, 33, 1121-34.
Glass, B. P. (1982) Tektites. In Introduction to Planetary Geology, pp. 145-172. Cambridge University Press, Cambridge.
King, E. A. (1977) The Origin of Tektites: A Brief Review. American Scientist, 65, 212-218.