Moldavites & Tektites
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.
Moldavite is an olive-green or dull greenish vitreous substance possibly formed by a meteorite impact in southern Germany, which would make it one kind of tektite. They were introduced to the scientific public for the first time in 1786 as “chrysolites” from Týn nad Vltavou in a lecture by professor Josef Mayer of Prague University, read at a meeting of the Bohemian Scientific Society. Zippe first used the term “moldavite” derived from the town of Moldauthein - now in Bohemia, from where the first described pieces came.
In 1900, F. E. Suess pointed out that the gravel-size moldavites exhibited curious pittings and wrinkles on the surface, which could not be due to the action of water, but resembled the characteristic markings on many meteorites. Boldly attributing the material to a cosmic origin, he regarded moldavites as a special type of meteorite for which he proposed the name of tektite. However, for a long time, it was generally believed to be a variety of obsidian. Because of their difficult fusibility, extremely low water content, and its chemical composition, the current overwhelming consensus among earth scientists is that moldavites were formed about 14.7 million years ago during the impact of a giant meteorite in present-day Nördlinger Ries. Splatters of material that was melted by the impact cooled while they were actually airborne and most fell in Bohemia. Currently, moldavites have been found in an area that includes southern Bohemia, western Moravia, the Cheb Basin, Lusatia, and Waldviertel. Isotope analysis of samples of moldavites have shown a beryllium-10 isotope composition similar to the composition of Australasian tektites and Ivory Coast tektites. Their similarity in beryllium-10 isotope composition indicates that moldavites, australites, and ivorites consist of near surface and loosely consolidated terrestrial sediments melted by hypervelocity impacts.
99% of all moldavite finds have come from the South Bohemian localities, 1% were found in South Moravian localities. Only tens of pieces were found in the Lusatian area, Cheb basin area and Northern Austria. Principal occurrences of moldavites in Bohemia are associated with Tertiary sediments of the České Budějovice and Třeboň Basins. The most prominent localities are concentrated in a NW-SE strip along the western margin of the České Budějovice Basin. The majority of these occurrences are bound to the Vrábče Member and Koroseky Sandy Gravel. Prominent localities in the Třeboň Basin are bound to gravels and sands of the Domanín Formation.
In Moravia, moldavite occurrences are restricted to an area roughly bounded by the towns of Třebíč, Znojmo and Brno. The colour of Moravian moldavites usually differs from their Bohemian counterparts, as it tends to be brownish. Taking into account the number of pieces found, Moravian localities are considerably less productive than the Bohemian ones; however, the average weight of the moldavites found is much higher. The oldest (primary) moldavite-bearing sediments lie between Slavice and Třebíč. The majority of other localities in southern Moravia are associated with sediments of Miocene as well as Pleistocene rivers that flowed across this area more or less to the southeast, similar to the present streams of Jihlava, Oslava and Jevišovka.
The chemical formula of moldavite is SiO2(+Al2O3). Its properties are similar to that of other types of glass, and its Mohs hardness is 5.5. Moldavite can be transparent or translucent with a mossy green color, with swirls and bubbles accentuating its mossy appearance.