The epansion coefficient in the low temperature range is very similar to the expansion of obsidians (G.Heide & Müller 1999).

The industrial produced glasses are distinguished in generally from the natural glasses especially by the alumina content. The most important industrial glasses are the soda-lime-silica glasses. The SiO₂ content is in the ranges from 70 to 73%, exeptionally reacing to and can be as high as 79% (so called „very hard glasses). „Hard glasses" are defined by the expansion coefficient < 6.10^-6 K^-1. The tektite glasses are are determined thereforein this terms as „hard" up to „very hard" glasses.

The partially replacement of the three basic oxides Na₂O, CaO and SiO₂ by oxides of other equivalent elements or at least by those with the a similar ion radius will it is possible to formulategive a hard glass alakli-lime-silica glass inwith the formula

(Na₂O, K₂O) (CaO,MgO, BaO) (Al₂O₃, B₂O₃) (SiO₂)

In this conception acts the alkaline and earth alkaline oxides act as a network modifier, silica and boron as the network former and alumina as intermediate oxide. By Stevels (1960) was introduced a structure parameter Y, which represents the mean value of the number of bridging oxygens per [SiO₄]-unit. Pure silica glasses is characterized for example by Y = 4. Y decreases Wwith increasing the network modifier content Y decreases. Glasses with Y<3 should not be interpreted in terms of the network hypothesis. In the case of Y< 2 , thatwhich means the SiO₂-content is less than< 50% „invert glasses" results. In this glasses the bonding strengths between the modifier cations and the oxygen ion becomes determineding for the glass porperties. In tectites and obsidians is Y is greater than 3 (Y > 3) which results into a and a three-dimensional Si-O-network which determines theits glass structure.As was shown the differences between the tektite and obsidian glass structure is given by the intermediate structural order (G.Heide & Müller 1999).

Generally the crystallization rate and the crystall growth decreases or even stopeds if different cations of varying size and charge are introduced.

Aluminium oxide is a most important additional component additive. If Al₂O₃ is introduced into a silica glass, the Al³⁺-ion is given the opportunity to form a [AlO₄]- tetrahedron which acts like the [SiO₄]-tetrahedron as a network former. If , however, the molar ration R₂O : Al₂O₃ is less than 1 < 1 the valence compensation available to the Al³⁺ ions in the tetrahedral coordination is no longer possible and a [AlO₆] octahedral coordination results.

Compositions with the molar ratio R₂O : Al₂O₃ = 1:1 resp. In geoscinces special interesting cases are compositions with the mole ratio R₂O : Al₂O₃ = 1 : 1 resp.RO : Al₂O₃ = 1 :2, are of special interest in geosciences. tThat means the formation of aluminosilicate melts and feldspar crystalls, e.g. in a orthoclase melts results the concentration of SiO₂ = 64,7%, Al₂O₃ = 18,4% and K₂O = 16,9% resp. in albite SiO₂ = 68,8%, Al₂O₃ = 19,4% and Na₂O = 11,8% will result.

The coordination of alumina in tektite glasses and obsidian shouldmay be not only be in a tetrahedricalon coordination, but determination of the coordiantion numbers in such complex systems like tektites and obsidians have not been determinedis at present a open question in my opinion.

A generally favourable influence of Al₂O₃ on the properties of glasses occurs in soda —lime-silicate glass melts by the additions of 2 to 3Mwta% Al₂O₃ but below 4 Mawt% (Volf 1990). Particulary results The addition of Al₂O₃ an increasinges the chemical durability and suppressing the crystallization rate. Introduction of Al₂O₃ causes an increasinge in viscosity at all temperatures. Experimental results are available for binary silicate melts with a replacement of 20 Mawt% SiO₂ by Al₂O₃. The introduction of Al₂O₃ acts in this casehas a similar effect like a decrease of thein alkali oxide content. This effect is only possible as long as alkali ions are present for the valence compensation of Al³⁺.

The differences between obsidians and tektites in the composition of the major elements is small, but generally tektites have higher MgO for given SiO₂- content and in contrast to many obsidians K₂O is greater than Na₂O. Furthermore, the water content is much lower in tektites than in terrestrial volcanic and industrial produced glasses.

The iron in tektites is more reduced. The redox state of iron is essentially allto a great extend Fe²⁺ (Schreiber et al.1984).

The effect of an addition of MgO into a simple silica melt with CaO is a reducingtion in of viscosity and suppresses the devitrification. An optimum effects was observed up to 4 Ma% MgO (Volf 1990).

Substitution of MgO and Al₂O₃ by FeO (1,5 — 10,5 Mol%) and Fe₂O₃ (0,8 — 3,1 Mol %) in a cordierite like glass shows a increasinwideningg of the inmiscibility gap and the crystallization of a ferrimagnetic phase was observed (Vogel 1992). Mössbauer measurements have shown that [Fe³⁺O₄] groups occur in silicate glasses, whereby the Fe³⁺ ion behaves like the Al³⁺ ion, but octhedral [Fe³⁺O₆] groups are also assumed. The position of Fe³⁺ in the 6-field coordination is strongly determined by the glass composition (Scholze 1990).

Fe²⁺ was observed in reducing melts in the form of oktahedral [Fe²⁺O₆] groups. The redox state is controlled by temperature and oxygen partial pressure and given by the following reaction

2O^2- + 4Fe³⁺ <----> 4Fe²⁺ + O_2(g)

Summarizing the different aspects of chemistry in tektite and obsidian melts weone must consider that only small chemical differences must are be responsible for the obviously differences in the properties of the both two glass types e.g. content of crystalline phases, water content, redox state. Theseis results underline that the discussion of the physical evidence could be very important for the understanding of the origin of these glasses origin.

An imporotant fact in the discussion about the origin of tektites is the „homogenity". O’Keefe wrotes in his review (1984) „Under the microscope they (the tektites) are seen to be homogeneous, and to lack microlites or crystallites (p.9). This remark is surprising because under the microscope are shwon a very there are a rather inhomogneous glass with bubbles, strias and glassy inclusions (Heide 1989).

By examinationing of tektite glasses with different methods (electron microscopy, electron microbeam probe, optical microscopy and X-ray diffraction) were found a different degree of chemical and phase homogenity resp. heterogenity was found (McPherson et al. 1984).

From the viewpoint angle of known glass forming processes and the knowledge about the influence of chemical composition, and the melting history on the microstructure and the physical properties we can discusse the different features in the microstructure. AsIt is is well known there are big differences in the glass forming processes in nature and in industry ( Klöß,Heide 1998)

V. Engelhard et al. (1987) have been discusseded the remarkable homogentiy of the chemical composition of moldavites in relation ofto the extremly melting conditions during the generation origin of moldavites. After this tThe initially ionized gas clouds (temperatures of some 10.000°C and pressures of about 500 Gpa) wasere hurled from the impact point in probably several jets and separated by a rapid drop of temperature into a liquid condensate and a vapor phase. Lechatelierite grains in moldavites were quartz grains incorporated into the jets. The cooling rate of the glass droplets was determined by Arndt and Rombach (1976) to be between 2 and 20 K/sec and more recently by Klöß (1999) up to 1000 K/sec. By Klöß also was determined by thermal modified density measurements the extremly different thermal historyies of tektites and obsidians by thermal modified density measurements. The hypothesis of v.Engelhard et al. that the formation of moldavites occurs by with a condensation from a plasma of vaporized sands could be solved the problem of homogenity and the chemical differences between different strewn fields in a relatively small geographic area and should be an alternativ hypothesis to the lunar volcanism by O‘Keefe .

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Futrell D.S.: The lunar origin of tektites, Rock & Gem Feb.1999, 40 - 43

Engelhardt v.W., Luft E., Arndt J., Schock H., Weiskirchner W. : Origin of moldavites; Geochim.Cosmochim.Acta 51 (1987) 1425 —1443

Glass B.P.: Tektites; J.Non-Cryts.Solids 67 (1984) 333 — 344

Heide G., Müller B.: Zur Struktur von Moldavitglas, Schr.Staatl.Museums Min. Geol. Dresden Nr. 10 (1999), 30 - 33

Heide K.: Structural features of natural glasses and their correlation to the glass foming processes; Chemie d. Erde 49 (1989) 287 — 295

Hölzle-Vuynovich A.: Petrographische und geochemische Untersuchungen an Schneeflockenobsidianen und verwandtem Material aus den USA, Island und der Osterinsel; Heidelberger Geowiss. Abh. 56 ( 1992) Universität Heidelberg

Izokh E.P.: Origin of tektites: An alternative to terrestrial impact theory; Chemie d.Erde 56 (1996) 458 — 474

Klöß G., Heide K.: Oriented crystallisation in natural glasses Glas Techn.Ber.GlassSci.Tech. 71c (1998) 397 — 399

Klöß G., Heide G.: Geospeedometrie an Tektiten, Schr.Staatl.Museums Min. Geol. Dresden Nr.10 (1999) 52 - 54

Klöß G., Heide K.: Geospeedometrie natürlicher Gläser; Beih.Eur.J.Min. 11 (1999) 126

Lange J.M.: Lausitzer Moldavite und ihre Fundschichten; Schriftenr.f.Geowiss 3 (1995) 7 — 138

Luft E.: Zur Bildung der Moldavite beim Ries-Impakt aus tertiären Sedimenten; Enke Verlag 1983 p.57

McPherson D., Pye L.D., Frechette V.D.: Microstructure of natural glasses

O’Keefe J.A.: Natural glasses; J.Non-Cryst.Solids 67 (1984) 1 — 17

Rost R.: Vitaviny a tektity; Czech.Acad.Sci.Publ. Academ.Press Prague (1972) p.241,

Schreiber H.D., Minnix L.M., Balazs G.B.: The redox state of iron in tektites; J.Non-Cryst. Solids 67 (1984) 349 - 359

Scholze H. : Glass p.236

Schnetzler C.C., Pinson jr. W.H.: Tektites ed. J.A.O’Keefe (1963) Chicago Press p. 101

Stevals J.M.: Neue Erkenntnisse über die Struktur des Glases; Philips‘ tech.Rdsch. 22 (1960/61) 337 — 349

Vogel W. : Glaschemie, Springer Verlag (1992) p.371

Volf M.B.: Technical approach to glass; Elsevier (1990)