EXTREMELY HETEROGENEOUS MUONG NONG TYPE MOLDAVITES. L. Švardalová 1 , R. Skála 2 , M. Trnka 3 , S. Houzar 4 , M. Novák 1 . 1 Department of Geological Sciences, Faculty of Sciences, Masaryk university, Kotlářská 2, 611 37 Brno, Czech Republic, e-mail: 78338@mail.muni.cz. 2 Institute of Geology, Academy of Sciences of the Czech Republic, Rozvojová 269, 165 00 Praha 6, Czech Republic. 3 Lithos Co. Ltd., Durďákova 41, 613 00 Brno, Czech Republic. 4 Department of Mineralogy and Petrography, The Moravian Muzeum, Zelný trh 6, 659 37 Brno, Czech Republic. Introduction: Moldavites (the Central European tektites) represent natural glasses which belong to a group of similar materials from other regions (North American, Ivory Coast and Australasian) called tek- tites [1]. Recently, tektites have been generally inter- preted as products of terrestrial impact events. Mol- davites are believed to be formed by the Ries impact in southern Germany 14.3–14.8 Ma ago [2, 3]. They are deposited at distances of 200 km to 450 km from the source crater. Moldavites are found in southern Bohe- mia, southwestern Moravia, Cheb Basin (Czech Re- public), in Waldviertel (Austria) and Lusatia (Ger- many). They were formed from sandy and clay Terti- ary sediments that covered the surface of the Ries im- pact site [4, 5]. There are some regional variations in bulk chemi- cal composition among moldavites. In the Bohemian area the composition is rather uniform, but some mol- davites from northernmost Radomilice area are higher in SiO 2 and lower in all other major oxides. Moravian moldavites are higher in Al 2 O 3 and FeO and lower in CaO than the average of Bohemian ones. On the basis of their morphology, texture and chemical composition, tektites can be divided into two groups: (1) splash form and (2) Muong Nong type. The splash form tektites (SF) are relatively homogene- ous. Muong Nong (MN) type tektites are layered, fragmental in shape and they do not display aerody- namic shapes), show wider heterogeneity in texture and chemistry, contain more bubbles, have higher con- tents of volatile elements and contain relic minerals [6]. All these differences are indicative of formation under lower temperatures than SF tektites. Wider chemical variability of MN type tektites may be inter- preted as an incomplete mixing of several end- members of parent rocks [7]. The occurence of the MN type tektites among moldavites is still intensively de- bated. Samples and methods: Nineteen samples of mac- roscopically heterogenous moldavites and four typical SF moldavites from three localities in southern Bohe- mia (Slávče near Trhové Sviny, Besednice, Dobrkov- ská Lhotka) were studied. Macroscopic and optical properties (optical microscope) of all specimens were studied. Chemical data of thirteen MN type moldavites and two SF moldavites was collected using a CAMECA SX100 microprobe. Accelerating voltage was 15 kV, sample current 15–20 nA, and electron beam diameter 5 μm. Following elements were ana- lyzed (using the standards in brackets): Si (augite), Ti (titanite), Fe, Ca (andradite), Mn (rhodonite), Na, Al (albite), Mg (olivine), K (sanidine), Zr (zircon), Ba (benitoite), P (fluorapatite). Macroscopic and microscopic characterization: The MN moldavites are usually darker in colour (brown, browngreen) in opposite of SF moldavites. With increasing heterogeneity they become less trans- parent. They display irregular fragment-like appear- ance. They show distinct layering or they have linear sculpturing. Some samples are bent. Sometimes nee- dle-like bubbles were observed. Glass of two different texture – porous and compact – was identified to occur simultaneously in four samples. Fig. 1: Heterogenous MN type moldavites with porous and compact layers (1 division = 1 cm). The MN type moldavites are much more optically het- erogeneous than the SF ones and much their fluidal structure is more pronounced. Alternating layers are almost parallel to each other in these moldavites or they are irregularly contorted. Parallel layering is lo- cally highlighted by elongated inclusions of lechatel- ierite or bubbles. Bubbles and lechatelierite (of vari- able size and shape) are more frequent in MN type moldavites. Chemical composition: The range of major ele- ment composition is significantly wider in MN type moldavites than in SF moldavites. This chemical het- erogeneity is illustrated in Table 1 where the data for MN and SF type moldavites are compared. Data com- parison of compact (C) and porous (P) glasses of one sample of MN type moldavite shows that compact Lunar and Planetary Science XXXIX (2008) 1962.pdf