Original paper

Zusammensetzung und Ursprung von Wüstengläsern aus der Großen Sandsee Ägyptens

[Composition and Origin of Libyan Desert Silica Glass from the Great Sand Sea of Egypt]

Jux, Ulrich

Kurzfassung

Braunstreifiges und dunkles Kieselglas (LDSG) aus der Westlichen Wüste Ägyptens (Große Sandsee) enthält Kohlenwasserstoffe mit starker Beteiligung isoprenoider Verbindungen. In Einschlüssen sowie in Mazeraten des LDSG konnten neogene Mikrofossilien (Sporomorphe) erkannt werden. Die daraus zu folgernden irdischen Bildungsprozesse des LDSG stehen auch mit anderen Analysenbefunden in Einklang. Sie finden ihre geologische Bestätigung in den primären Lagerungsverhältnissen; denn das nicht korradierte Wüstenglas kommt mit rauhen, sandigen Krusten auf Spalten (Schrumpfrisse!) in neogenen lakustrinen Ablagerungen vor. Deshalb geht die Bildung von LDSG auf Überschichtungen alkalischer Porenwässer durch saure Seewässer zurück, wobei der Koagulation von SiO2-Gelen die Korrosion biogener Skelettopale vorausging.

Abstract

The chemical composition of silica glass (LDSG) from the Western Desert of Egypt (Great Sand Sea) is characterized by the high content of SiO2 (~ 98%), considerable concentrations of Al2O3 (~ 1 %) yet only small amounts of alkalies and earth alkalies (0,01-0,03 %). New analytic results confirm a rather uniform composition of all LDSG samples studied. As a rule water contents are rather high (~ 0,1 %). Most of the corraded or otherwise polished desert glass has a greenish-yellowish, slightly opalescent transparency. Except some weak reflexes of Cristobalite and Tridymite the x-ray diffraction pattern reveals an amorphous composition that lacks definite crystalline structures. There are just a few dark or light varieties with varying amounts of included Cristobalitic spheres (~ 1 mm), thus showing tendencies for ordered internal arrangements. Brownish or dark LDSG includes both saturated and unsaturated hydrocarbons which are marked by a noteworthy share of isoprenoid compounds. Neogene microfossils, mainly plant tissues and sporomorphs, could be identified from macerated samples as well as in chips (artefacts!) and thin sections. From this follows a terrestrial origin of LDSG. This agrees well with other analytical results, especially of the gas extracted from bubbles in milky glass. Geologically this conclusion is confirmed by primary depositional occurrences of LDSG. With its coarse, sandy crusts, desert glass is found in fissures (desiccation cracks) of neogene, lacustrine deposits (gravel, sands, clays). The adjacent rock shows no zone of reaction, however, it coined the original sculpture of LDSG, its shape being defined by the crevices. During a warm-arid climate the processes of diagenetic formation of desert glass were bound to a terminal lake basin, which suffered alternating humid and dry phases. The influx of renewed freshwater with relatively low pH-values (biogene CO2) covered brackish residual water bodies (ground-water) with relatively high pH-values. Through desication cracks, groundwater and lakewater came into contact and there, colloidal solutions of SiO2, which derived from the corrosion of biogene silicious skeletons, coagulated. Thus, LDSG emerged from the colloidal mobilisation and final gelation of silica and is neither related with lechatelierites nor with tektites.

Keywords

Silica glasshydrocarbonsisoprenoidchemical compositioninclusionsporomorphsNeogenegenesisincrustationfissurelacustrine sedimentcorrosionbiogene silicious skeletonscoagulationsilica gels Western Egyptian Desert (Great Sand Sea)