Moldavites: a review


Authors: Trnka M, Houzar S

Published in: Bulletin of Geosciences, volume 77, issue 4; pages: 283 - 302; Received 30 April 2002; Accepted in revised form 22 October 2002;

Keywords: tektite, moldavite, origin, physical properties, chemical composition, textures, Ries crater,

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Moldavites from southern Bohemia, from western Moravia, from the Cheb Basin, from Lusatia (Germany), and from Waldviertel (Austria) are the only known European tektites. In the present paper, we briefly sum up the existing knowledge about their strewn fields and geology, about their properties, and their origin. The present survey should enable a detailed comparison with other groups of tektites and separation of primary differences from differences caused by earth history. The extent of moldavite occurrences is a result of intensive denudation and redeposition of the initial strewn field. All regions of moldavite occurrences are spatially associated 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. Fluvial transport of moldavites to more distant places determined their present distribution and led to a substantial lowering of their content in the sediments. Roughly 106 metric tons of moldavite matter (macrotektites) formed initially. Only about 1% of this mass 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 but their amount could be much higher at the time of their formation. Micromoldavites were not found. Their preservation in the conditions of continental sediments over a time period of about 15 m.y. is not probable. It is, however, a question whether they were formed or not. Moldavites represent the most acid group of tektites with silica content of 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 lead to their higher translucency, similarly as in georgianites. In the same way as with other tektites, moldavites originated by fusion and ejection of porous target rocks during an oblique impact of a large meteorite. The impacting body - in the case of moldavites - was probably a chondrite 500-1000 m in diameter. Its impact also created the Ries crater at approximately 14.4-15.1 Ma.