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    Desert Glass

Steve writes:

Libyan Desert Glass (abbreviated "LDG" and also known as Great Sand Sea Glass or the Libyan Gold Tektite) are tektites found on the floor of corridors between sand dunes in the southwestern corner of the Great Sand Sea. Thought to have formed 29 million years ago when a meteorite hit somewhere around what is today the border between Western Egypt and Eastern Libya, the strewn field covers a 30- by 80-mile area, with the highest concentration of pieces found in a central ring-shaped zone about 3 miles in diameter.

LDG was formally "discovered" in 1932 by geographer Patrick Clayton while on a scientific expedition in the Libyan Desert, but it had actually been known to the native people for centuries, being used as talismans and for tools. In fact, the most famous example of the ancient use of LDG is the scarab in King Tutankhamun’s pectoral, though only in 1998 – after decades of puzzling archaeologists because the ancient Egyptians weren’t known for being able to create high quality glass – did Italian mineralogist Vincenzo de Michele analyze the optical properties of the gemstone in King Tutankhamun’s breastplate to prove it was carved from a piece of LDG.

While a few boulders of LDG have weighed as much as 8 kilos, most pieces are smaller than one’s fist. Usually golden colored from trace inclusions of iridium, LDG is comprised mostly of lechatelierite – an amorphous, non-crystalline mineraloid form of SiO₂ (mineraloids differ from true minerals in that mineraloids – while naturally occurring like minerals – do not exhibit crystallinity and have chemical compositions that vary beyond the generally accepted ranges for specific minerals like quartz, where lechatelierite is often grouped).

Experiments have shown that enormous shock pressures are required to create lechatelierite, with it forming terrestrially when quartz sand is melted from lightning strikes or from meteorite impacts. (Two other famous occurrences – one artificial and one natural – are, respectively, lechatelierite that formed within the trinitite created by the first nuclear bomb explosion at Trinity Flats in What Sands, New Mexico, and lechatelierite created at Arizona’s famous Barringer Meteor Crater where, during the rapid pressure reduction after the meteor’s impact with the Coconino Sandstone, steam expanded the newly formed mineraloid, creating an expanded and shattered glass with a density less than that of water.)

LDG measures about 5.5 on the Mohs hardness scale, and pieces not sandblasted by the desert winds exhibit a glassy luster. Some examples are translucent, while others are milky, bubbly, or even more amber-orange in color. Less common pieces can exhibit a greenish color or have dark streaks running through them.

There is some continuing debate regarding the origin of those dark streaks found in some pieces of LDG. One study concluded they contained carbonaceous material presumably from a stony impactor; a second described osmium abundances and isotopic values that suggest "a meteorite variable", with the authors also noting that in layers having higher iron content, the more depleted state of that iron implied it was from the original meteorite impactor. But in a third study, the authors concluded the pronounced correlation between the abundances of Cr, Mn, Fe, and Ni in their samples’ black material was clearly nonchondritic, suggesting it formed terrestrially; conversely, they found that the brown swirls seen in some pieces contained what appeared to be extraterrestrial minerals.

And a more recent study seems to all but rule out competing theories that LDG was formed by a high-altitude airburst from a meteor or terrestrial vulcanism. That study described the discovery in LDG of reidite – a mineral specific to meteorite impacts, as it only forms when extremely high pressures force zircon atoms into a tighter arrangement.

[Technically, reidite was not discovered in LDG, as it’s only preserved if the shocked rocks do not melt. When hot, reidite becomes unstable and reverts back to zircon at temperatures above 1200 °C. But the transformation to reidite occurs along specific axes in a zircon crystal, and when the reidite changes back to zircon, it leaves traces of its existence in the final orientation of the atoms. As a consequence, electron backscatter diffraction can be used to show that reidite once existed in shocked zircons that subsequently got hot and recrystallized.]

The high pressures and temperatures attendant with LDG’s formation created other unusual minerals, including 0.1 to 2 mm cristobalite spherules (a high temperature form of quartz found in some pieces of LDG). Additionally, these same extreme conditions created LDG’s baddeleyite (ZrO₂, also called zirconia) and silica glass (SiO₂), both forming when the zircon (ZrSiO₄) broke down at temperatures exceeding 1676 °C – hotter than for any igneous rock on Earth and consequently another indication that LDG is of impact origin.

But while these minerals can form in impact craters, no crater has yet been positively associated with the LDG strewn field, though the Libyan Kebira and Oasis craters – about 45 km apart – have each been proposed as possibilities. Unfortunately, Kebira has been deemed unlikely because its flat top indicates an undisturbed stratum, as opposed to an expected folded, tilted, and faulted one consistent with an impact site. Alternately, while Oasis is significantly eroded, its remaining structure is more consistent with an impact crater, and studies have shown some chemical and isotopic similarities between LDG and rocks found at that site; in addition, if one presumes an elevated original ground surface, then gradual erosion of the underlying bedrock would result in fragmenting of the glassy crust created by an impact, resulting in those fragments being distributed in a way thought consistent with the observed LDG strewn field.

However, in 2020, a third possible impact site – the 1000-foot wide El Bahr Crater – was found during a satellite imagery review of the terrain between the Egyptian villages of Qaret Had El Bahr and Qaret El Allafa; by combining various wavelengths of light from those images, researchers identified a much higher concentration of orthopyroxene in the crater‘s basalt compared to the surrounding rocks, suggesting it was melted by a high-temperature impact event, with large crystals of orthopyroxene forming from the slow cooling that followed. (Interestingly, the ancient Egyptian word for glass that’s preserved in hieroglyphic texts means "stone that pours".)

LDG is unique in that its olivine crystals have characteristics that have not been seen in any other tektites discovered so far. Similarly, LDG sand contains two types of magnetic particles not found anywhere else, and the tektites tend to have higher concentrations of iron oxide compared to other tektites found around the world, as well as smaller amounts of magnesium oxide and silicon dioxide.

I’d guess that many (if not most) of Paul’s readers have at least one example of LDG in their collection. Over the years, I’ve acquired several smaller pieces of varying quality, a few of which even show evidence of being worked and used as tools. But I only have two larger examples (Photo 1). The first weighs just over 700 grams and measures roughly 130 x 130 x 70 mm (Photo 2 shows its two largest surfaces). I bought it at the 2006 Tucson show where a Russian vendor was selling out of a corner of a room that his friend had rented. The Russian’s glass was not as gemmy, and consequently he was selling larger pieces for substantially less per gram. And some – including the one I purchased – had the "undesirable" (at the time) gray and tan inclusions that he claimed were meteoritic material (this was the first time I’d heard of that or seen any LDG with these dark streaks, which you can see in Photo 3). This piece also has several "tubes" formed presumably by hot gasses, with one protruding entirely through the length of the piece (Photo 4). And while not particularly translucent, it still is quite interesting when backlit (Photo 5). So despite its flaws, it was cheap and I took the chance, only to be pleased later when I read papers about the likely source of the dark streaks it contained.

My most recent acquisition is not as large – 386 grams and approximately 130 x 75 x 60 mm – but except for not being worked, it has about every other feature I’m aware of in LDG: excellent translucency, clearly-defined areas with dark material, both naturally sandblasted and non-altered areas, several inclusions of cristobalite, and a natural hole along with two other "tubes" that almost pass completely through (Photo 6 shows this piece’s two largest surfaces; the clear boundary between areas with and without presumably meteoritic material and its natural hole is shown in Photo 7). Like the first piece, this one is also interesting to view when backlit (Photo 8).

The LDG strewn field is expansive (Photo 9), and large amounts of the tektite continue to be found. While prices rose during the pandemic due to quarantines that limited collection, they’ve recently come down as availability is returning to pre-pandemic levels. Consequently, I doubt my collection of LDG is complete . . .