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Moldavite - what is it?

Moldavites represent a group of tektites subjected to a number of relatively detailed studies. On the other hand, moldavites have been strongly influenced by their geological development. They are found in several discrete regions: southern Bohemia, western Moravia, the Cheb Basin, Lusatia in Germany, and Waldviertel in Austria. Only a few moldavites were found outside these regions.

The extent of moldavite occurrences is a result of intensive denudation and redeposition of the initial strewn field by surface streams. All regions of moldavite occurrences are spatially connected with regional basins and depressions. The oldest moldavite-bearing sediments with very short-transported material are unsorted colluvio-fluvial gravelly sands and clays of Middle to Upper Miocene age. They occur especially in the South Bohemian substrewn field. In Moravia and perhaps in Austria, they are rare. Fluvial transport of moldavites to more distant places
determined the present distribution of moldavite occurrences and led to a substantial lowering of their content in the sediments.

Roughly 106 metric tons of moldavite matter (macrotektites) formed at the time of their formation. Only about 1% of this matter has been preserved till the present.

Most moldavites are splash-form moldavites. No ablation features were found on their surface. Muong Nong type moldavites occur sporadically. It is possible that in the time of origin their amount was higher. Micromoldavites were not found. Their preservation in the conditions of continental sediments over about 15 million years is not probable. It is a question whether they were formed or not.

Moldavites represent the most acid group of tektites, with silica content around 80 wt%. They are relatively rich in K2O, too. On the other hand, they are characterized by low average  contents of Al2O3, TiO2, FeO and Na2O. These low contents of TiO2 and FeO are responsible for their higher translucency, similarly as in georgianites.

In the same way as with other tektites, moldavites originated by the fusion and ejection of surface rocks during an oblique impact of a large meteorite. The impact body in the case of moldavite formation was probably a chondrite 500–1000 m in diameter. Its impact, dating to 14.4–15.1 Ma, created also the Ries crater.

 

Rewritten from Bulletin of the Czech Geological Survey, Vol. 77, No. 4, 283–302, 2002.