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The characteristic feature of all tektites is their specific and rather uniform chemical composition (except for some microtektites), in which they differ from other glasses
of natural and artificial origin. The affinity with some sediments follows already from the main oxide contents (the high Al2O3/K2O + Na2O ratio, the high content of
CaO and MgO). Bouška (1968) described the affinity between the sediments and moldavites.
Also, the contents and the ratios of trace elements indicate that (i) moldavites differ from meteorites, lunar rocks and terrestrial igneous rocks and that (ii) they are close to clay- and sand-dominated sediments (for example, the distribution of REE, the ratios of Zr/Hf, K/U, Th/U, K/Rb and others – Bouška 1992). No indication of contamination
by meteoritic material was found whatsoever.
In spite of the generally close chemical composition of tektites coming from various places on the Earth, certain differences can be also found between them. The moldavites
form the most acid group of tektites, with SiO2 contents about 80 wt%. They are also relatively rich in K2O. On the other hand, they have very low average contents of
Al2O3, TiO2, FeO and Na2O. North American tektites are the nearest to moldavites, especially the georgianites, which are very close to moldavites from many points of view as well (Bouška et al. 1990b, Koeberl 1990).
Minor differences exist even among moldavites. Table 2 shows the average moldavite compositions from various parts of strewn fields. The fluctuation of individual oxide
contents can be explained by variable contents of three essential mineral components in the parental rock, i.e. quartz, clay minerals and carbonates (Delano et al. 1988 –
see below). From this point of view, the source material of the Bohemian moldavites was relatively rich in carbonates, while it was clay minerals in the Moravian moldavites
and quartz in moldavites of the Radomilice group.
Most chemical analyses show the overall composition of the moldavites. The inhomogeneity of individual pieces, which is obvious also from the existence of fluidal
texture, is caused by local fluctuations in chemical composition. The extent of these fluctuations approximately corresponds to differences among various moldavites
(Cílek et al. 1992). Engelhardt et al. (1987) determined the following variation in the chemical composition of a moldavite from Ločenice (southern Bohemia) from 20 point
analyses on a line 0.27 mm long: SiO2 78.6–82.7 wt%, CaO 1.8–2.7 wt%, MgO 1.3–1.8 wt%, FeO 1.15–1.67 wt%, Na2O 0.30–0.45 wt%, K2O 3.5–3.9 wt%.
The extremely low content of most volatile components is characteristic of all tektites. The results of Beran and Koeberl (1997) show that the tektites have water content
ranging from 0.002 to 0.030 wt%, and moldavites themselves from 0.006–0.010 wt%. These results show that tektites are in general the driest natural material. During moldavite formation, vaporization of a high proportion of rare gases, halides, sulphur, carbon and nitrogen from the source occurred (Moore et al. 1984, Bailey
1986, Matsuda et al. 1993, Meisel et al. 1997 and others.). A distinct decrease also occurred in less volatile components, for example As, Sb, Cu, P.