Libyan Desert Glass
This gemstone shown in the picture is 28.5 million year old silica desert glass, found in-between sand dunes on the border of Libya and Egypt in the Great Sand Sea. People used the glass in the pleistocene era to make tools and carvings, and since its scientific discovery and investigation began in 1932, researchers have been trying to find an origin, but ultimately the reason has remained a mystery.
Color varies from transparent, milky white, light yellow, to a light green or green-brown color. The pieces vary in size and mass, and are often sand and wind eroded smooth on one side, with slight etchings resembling pseudo-regmaglypts, or characteristic grooves like a meteorite, on the other side. This glass contains high levels of silica, at around 98%, and can include many bubbles, dark brown streaks, and/or cristobalite spherulites, which are high-temperature silica structures.
Trace elements found in dark streaks within the glass include iron, magnesium, and aluminum, while wisps of iridium and iron in a low oxidation state point to an extraterrestrial origin for these impurities.
Scientists thought that the glass was formed by the impact of an extraterrestrial body into a sandstone layer, but no crater was found. This led researchers to hypothesize that a low-altitude airburst by a meteorite could supply the necessary heat to melt the sandstone, forming the gems we see today. This tektite would be similar to Trinitite, a glass made from the desert sands in New Mexico during the atomic bomb trial (see this Earth Story post on Trinitite: http://on.fb.me/1JV0IWh ).
A possible crater named Kebira was discovered in 2007, though very little research has been published regarding this site. A study did investigate in 2011, analyzing oxygen isotopes of sandstone taken from glass-containing areas and the proposed Kebira zone. This study determined that the glass did not originate from the proposed impact crater, but that did not identify an alternative source.
In 2013, scientists reported on a meteorite called “Hypatia,” a very hard, black, shiny, carbonaceous material with many fractures and diamond inclusions. Tests suggested that components in this rock formed at temperatures comparable to those found in liquid nitrogen, possibly suggesting that this stone is some of the only comet material found on Earth. Though there are some questions, the team suggests that a cometary nuclei entered Earth’s atmosphere, lost its water content and volatile hydrocarbons while gaining atmospheric noble gases, and then the bolide exploded. A diamond similar to the ones found in Hypatia has been found embedded in Libyan desert glass, possibly suggesting that a comet exploding in the atmosphere was the original source for this material.
On the other hand, a more recent study examined grains found in brecciated rocks associated with the Libyan Desert Glass strewnfield. In those rocks they found minerals like shocked quartz, which require large pressures - up to 20 GPa - to form in the state they were observed in. These pressures were unilkely to be produced during an airburst, leading those authors to again suggest that the Libyan Desert Glass was produced during an impact, but that the remnant crater has been completely eroded away.
-MH
Image Credit: James St. John, http://bit.ly/1zZYj75 References: Aboud, T., 2009, Libyan desert glass: Has its enigma been solved?: Physics Procedia 2, (2009) p. 1425–1432. Kramers, J.D, Andreoli, M.A.G., Atanasova, M., Belyanin, G.A., Block, D.L., Franklyn, C. Harris, C., Lekgoathi, M., Montross, C.S., Ntsoane, T., Pischedda, V., Segonyane, P., Viljoen, K.S., and Westraadt, J.E., 2013, Unique chemistry of a diamond-bearing pebble from the Libyan Desert Glass strewnfield, SW Egypt: Evidence for a shocked comet fragment: Earth and Planetary Science Letters, v. 382, p. 21-31. Longinelli, A., Sighinolfi, G., Michele, V.D., and Selmo, E., 2011, O and chemical composition of Libyan Desert Glass, country rocks, and sand: New considerations on target material: Meteoritics & Planetary Science , v. 46, p. 218-227. https://gsa.confex.com/gsa/2018AM/webprogram/Paper322276.html