Libyan Desert Silica Glass

Tutankhamun's pectoral, Jon Bodsworth, Wikimedia Commons

This mysterious material was ‘rediscovered’ in December 1932 by Patrick Clayton whilst leading an expedition for the Egyptian ‘Desert Surveys Department’ across the Great Sand Sea of the western desert, near the Libyan border. It was found lying around in some quantity in the open ‘streets’ between ranks of high longitudinal dunes and has since been located across a rectangular area the size of Devon! Mentioned  scientifically by Fresnel as early as 1848, it was also known to the ancients, as it turns up in Tutankhamun’s treasure as a carved scarab centrepiece, as well as worked artefacts in the archaeological record. It’s most common appearance is a greenish-yellowish ‘plastic-looking’ transparent rock, much of which has been worn smooth and polished by desert abrasion. Buried material however retains a jagged and angular shape, may often be frothy due to bubble inclusions and may also be bluish-grey through carbonaceous impurities. It is essentially 98% silica, 1.5% alumina with small amounts of alkalies and water. This sets it well apart from all other occurrences of natural glass such as volcanic rock, tektites and the also rather odd ‘Darwin Glass’ from Australia.

Glass Weight 22g, width about 55 mm, Wikimedia Commons, H. Raab

Not surprisingly, this glass has been much talked and written about, with whole expeditions dedicated to its study alone. Some authors have reported banded varieties in which dark layers have elevated ‘unearthly’ iridium contents; others report dust particle inclusions of certain elements in proportions similar to chondrite meteorites. Studies of gas bubble inclusions report their having proportions of inert gases, as well as beryllium and aluminium isotope characteristics, consistent with an earthly atmospheric origin. Just one (it seems) research team has claimed the inclusion of Neogene microfossils (mainly of plant affinity).

One of the few points of general consensus now appears to be the age of this material. At first dated around 26-29 Ma (million years), a fissiontrack age of 33.8 Ma is now widely reported. A concordance between the composition of the glass and the surrounding Nubian Sandstone bedrock is also seemingly established without demur, even down to such niceties as the germanium content. Regrettably, however, there is still little consensus regarding its origin. Most theories seem to invoke an extra-terrestrial involvement, but this has to overcome the difficulty of there being no credible impact crater in the neighbourhood. This has been explained by removal through erosion – a shade suspicious for such a young age (there are plenty of much older impact craters still recognisable on earth today). A more credible alternative might be a Tunguska-like fireball event; a mighty atmospheric explosion associated with the break-up of an object from outer-space in the upper atmosphere. The most prosaic explanation has come from the group who report the enclosed microfossils as well as a relatively high water content. This is consistent in their eyes with a wholly terrestrial explanation of solidification of colloidal silica gel deposits formed from siliceous micro-organisms living in shallow lakes’. But if so simple… then why not more often?

Dotted area shows where the glass is found

Written by
Mike Allen
BCGS Member

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