Tektites are natural, dark-colored, rounded silicate glass particles several cm in size and
often aerodynamically shaped and are found in various areas referred to as strewn fields around
the world. Four tektite strewn fields are known in the Cenozoic: Australasian at 
0.77
Ma, Ivory Coast at 
1 Ma, Central European at 
14.7 Ma and North American at
34 Ma. Three major types of tektites are found: 1) the centimetric elongated, tear drop
shaped or rounded splash form tektites, 2) the ablated tektites which are re-melted splash form
characterized by a flange, and 3) the more massive Muong-Nong or layered tektites weighting
up to several kilos. Microtektites are submillimeter-size tektites found in deep-sea sediments
within the Australasian, Ivory Coast and North American strewn fields.
The origin of tektites was controversial for many years. Ample geochemical and petrological data now demonstrate that tektites are formed by the melting of terrestrial surficial sediments by hypervelocity impact (for a recent reviews of tektites see Barnes, [1990]; Glass, [1990]). The Ivory Coast and Central European tektites are associated respectively with the Bosumtwi crater in Ghana and with the Ries crater in Germany.
Recent findings of shocked quartz within tektite layers has strengthened even more the conclusion that tektites form by impact melting of terrestrial sediments. Glass and Wu [1993] identified shocked quartz and coesite at all sites where a well defined North American microtektite layer had previously been reported, and additionally in many cores from the Australasian strewn field taken within 2000 km from the supposed source area located on the Indochina Peninsula.
Koeberl and Shirey [1993] used highly sensitive negative thermal ionization techniques
to determine the abundance and isotopic ratios of Os and Re in Ivory Coast tektites. Because
the 
Re/
Os and
[4]
Os/
Os ratios in meteorites
are drastically different from continental crust values, the Re-Os
system may provide a sensitive tracer for the presence and quantification
of an extraterrestrial component in impact-derived material. This
pioneering study not only demonstrates that it appears possible to
detect small amounts of the impactor (
0.6 % in Ivory Coast
tektites) incorporated in the impact products but may also permit
quantification of target/impactor mixing during impact events.
Several terrestrial impact craters also contain glass formed by melting of the target rock during the impact of a meteoritic projectile. Detailed studies of impact glasses are rarer than for tektites and microtektites. Some of the best studied examples are the Zhamanshin crater glass in USSR, the Darwin glass from Tasmania, the Ries impact glass, the Bosumtwi crater glass, Aovelloul crater glass and Lonar crater glass. Most impact glasses show a broad range of chemical composition, even when analyzed on a micrometer scale, reflecting the composition of their parent material.