FMF - Friends of Minerals Forum, discussion and message board

[Roger Warin]

Hello,
Some topics that interest us will always raise the same questions.
For example, the Libyan Glass (LDG) is not a tektite at all, because its origin is totally different from a tektite. Its very existence is quite exceptional (only one case known).
Although we are all already on file with this new great god that is artificial intelligence (AI), I only asked ChatGPT one simple question, the one asked of the man on the street.
AI deeply disappointed me. It has a great memory but it didn't understand anything.
However, I am convinced that if I asked my question again in a more scientific way, perhaps in several stages, AI would give a better explanation.
I didn't take the test.

Distinction between Libyan desert glass and tektite ?
ChatGPT said:
Libyan desert glass and tektite are two types of natural glass, but they differ mainly in their origin and composition.
Libyan desert glass (or Libyan glass)
• Origin: This glass is formed by the fusion of sand and silica under extremely high temperatures, often caused by the impact of a meteorite or cosmic events. FALSE!
Libyan desert glass is mainly found in the Libyan Desert, but also in other regions of the Sahara. It is a type of glass that was formed during the impact of a meteorite, probably about 29 million years ago.
• Characteristics: Libyan desert glass is often golden or green in color and has a translucent texture. It consists mainly of silica, but may also contain traces of other elements such as aluminum or magnesium. POOR ANSWER CLOSE TO ERROR.
This type of glass often has inclusions and layered structures.
Formation: The temperature generated by the impact or other cosmic phenomena is such that it melts the sand, creating a solid glass. NOT AT ALL !!!
This process does not require the presence of a visible impact crater.
Tektites
• Origin: Tektites are pieces of natural glass also formed by the impact of a meteorite on Earth. However, they are found on a larger scale and are present in different regions of the world. Tektites can be produced by meteorite impacts but also by volcanic eruptions. They are formed by the fusion of rocks and silica under the effect of extreme temperatures and pressures.
• Characteristics: Tektites are often black, green or brown and have a smooth, shiny texture. They can have different shapes, often irregular or drop-like. Their chemical composition may vary slightly, but they typically contain potassium, aluminum, calcium and iron silicate.
• Formation: Tektites are generally formed during the impact of a meteorite that fuses the rock of the Earth's crust with extraterrestrial materials, thus creating a glass that solidifies after the impact. They are often smaller than the glass of the Libyan Desert and can be found hundreds or thousands of kilometers from the impact.
Main distinction
• Origin and location: Libyan Desert glass is specific to a geographical region (the Libyan Desert and the Sahara) and is often linked to a specific impact event. Tektites, on the other hand, can form anywhere in the world where a meteorite impact has occurred.
• Appearance and structure: Libyan Desert Glass is generally more translucent with golden or greenish hues, while tektites often have darker colors and a smoother, shinier texture. In addition, tektites can have a more regular shape (drop-shaped, spherical, etc.), while Libyan Desert Glass can have more irregular shapes and visible inclusions.
In short, although they are both types of glass formed by impact events, they differ in their place of formation, their appearance and their geological characteristics.

[Herwig]

Dear Roger,

Can you please let us know the scientific article(s) you based your message on?

Cheers, Herwig
-----

I don't know if you are aware of this fairly recent publication:
Elizaveta Kovaleva; Hassan Helmy; Said Belkacim; Anja Schreiber; Franziska D.H. Wilke; Richard Wirth
Libyan Desert Glass: New evidence for an extremely high-pressure-temperature impact event from nanostructural study.
American Mineralogist (2023) 108 (10): 1906–1923.

***
Abstract
The origin of Libyan Desert Glass (LDG) found in the western parts of Egypt close to the Libyan border is debated in planetary science. Two major theories of its formation are currently competing: (1) melting by airburst and (2) formation by impact-related melting. While mineralogical and textural evidence for a high-temperature event responsible for the LDG formation is abundant and convincing, minerals and textures indicating high shock pressure have been scarce. This paper provides a nanostructural study of the LDG, showing new evidence of its high-pressure and high-temperature origin. We mainly focused on the investigation of Zr-bearing and phosphate aggregates enclosed within LDG. Micro- and nanostructural evidence obtained with transmission electron microscopy (TEM) are spherical inclusions of cubic, tetragonal, and orthorhombic (Pnma or OII) zirconia after zircon, which indicate high-pressure, high-temperature decomposition of zircon and possibly, melting of ZrO2. Inclusions of amorphous silica and amorphous Al-phosphate with berlinite composition (AlPO4) within mosaic whitlockite and monazite aggregates point at decomposition and melting of phosphates, which formed an emulsion with SiO2 melt. The estimated temperature of the LDG melts was above 2750 °C, approaching the point of SiO2 boiling. The variety of textures with different degrees of quenching immediately next to each other suggests an extreme thermal gradient that existed in LDG through radiation cooling. Additionally, the presence of quenched orthorhombic OII ZrO2 provides direct evidence of high-pressure (>13.5 GPa) conditions, confirming theory 2, the hypervelocity impact origin of the LDG.
***

[Roger Warin]

Dear Herwig,
I was not familiar with this article, but all my previous readings came from scientists specializing in geophysics and astrophysics, but never in chemistry (or geochemistry).
All these authors only consider physical processes, with which I agree. I have never read an article in which other authors consider water as a chemical reagent even at these extraordinary temperatures and pressures.
Nor do I see any author (or rarely) being surprised that the SiO2 content in LDG is greater than 98%.
Where are these deposits of pure silica on the earth's crust? Only in diatom shell deposits. The creature extracted SiO2 from the ocean waters biochemically, I think.
In the case of the LDG, the fireball caused a chemical extraction of SiO2 from various silicate minerals, such as quartz, feldspars, inosilicates, etc., from local sandstones by water “molecules”. These originate either internally in the minerals impacted by the blast or in the water ejected from the comet. I favor the cometary origin.
The blast was caused by the impact of a comet on this point in Egypt, and it is impossible to say whether there was any actual contact with the ground, but it is probable.
In these extreme conditions of temperature and pressure, the water attacks the various crystalline structures of the rock, forming silanol groups that polymerize with each other. In particular, the SiO2 tectosilicate framework is broken by the introduction of a large quantity of silanol (Si-O) bonds. Water molecules are also captured. These will stabilize in various silica gels.
A comet hitting the Earth does not have the same effects as the fall of a stony asteroid, but rather that of an ice asteroid at -220°C (I think). It is also a hydrogen rocket, because the water molecules in the meteor's crust are transformed directly into a mixture of hydrogen and oxygen, in a kind of chemical sublimation (water decomposes at temperatures as low as 850°C and completely at 5000°C (at ordinary pressure) during the meteor's flight.
With these H2 + O2 leaks, several explosions already occur in flight, before touching the ground.
The LDG thus results from the action of this infinitely powerful blowtorch that struck the African savannah some 28-29 million years ago.
The presence of zircon reinforces the evidence of the existence of the high temperatures (especially) and pressures reached. It is therefore understandable that the role of water could not have been physical (no solution in water) but rather chemical in these Dantean conditions.
Then recalled the presence of zircon is not surprising because it is found in CAIs, one of the most refractory minerals, and also on some old massifs on Earth (dating to ~4.3 MA).
I am surprised that the team mentioned does not mention the presence of black streaks, residues of the stony (poorly aggregated) comet nucleus, including the famous presence of iridium.
What do you think about this, Herwig?

[marco campos-venuti]

I don't understand why you say that LDG was formed during a comet impact, but then you say that it is not a tektite. Considering that the other tektites are also the result of comet impacts. Please define tektite.

[alfredo]

Sorry, I just accidentally deleted someone's post here. If it was yours, please re-post it. The post appeared double on my screen, so I wanted to remove the duplicate, but they both disappeared. Must be a bug.

[Craig Hagstrom]

[Roger Warin]

Hello Marco,
Thank you for asking me this question.
A tektite is of terrestrial origin. Libyan glass is of terrestrial origin.
The difference is the impactor.
There are several types of asteroids:
1) Iron, which has good thermal conductivity. They do not shatter in flight.
The largest iron meteorite fell in Hoba (Namibia, 60 T), without making a large crater (tangential path), otherwise they explode on the ground (Meteor Crater) with the production of shards (Canyon Diablo meteorites).
2) Asteroids made of silicate rock, which are poor conductors, they shatter in flight and the meteorites (chondrites) are distributed over a fall field according to the Conservation of Momentum Law.
3) Asteroids made of ice (i.e. comets) with a temperature of around -225°C, with a core similar to the most primitive carbonaceous chondrites.
Composition: 80% water ice, 15% CO ice. The rest = CO2, CH4 and NH3.
+ primordial chemical molecules such as methanol, formic acid (giving formicaites), etc.
The whole thing is black because of space dust (all the polymorphs of carbon) and the oxides and silicates forming the CAIs (flaky refractory aggregates, the first minerals to appear in the nebula).

To understand the concept of tectite, it is best to give an example, a famous one that has been studied in great detail by German scientists.
Consider the asteroid that fell in Ries in West Bavaria about 15 million years ago.
This silicate asteroid exploded into two bodies:
the larger one is responsible for the Ries crater (near Nördlinger), Ø 24 km, and the smaller one fell in Steinheim (Ø 3.5 km).
With a slightly oblique angle of incidence, the Ries asteroid caused the ejection of splinters of vitrified rock. These ejecta followed a parabolic trajectory of 250 to 400 km in length, especially in Moldova, hence the name moldavites.
The aesthetic quality of this glass obviously comes from the nature of the impacted rocks. It is a great exception linked to the nature of the impacted soil, which is low in iron.
With, in addition, unmelted minerals: chromite, monazite, quartz, rutile.
And recrystallized or dissociated minerals: baddeleyite, coesite, corundum, cristobalite.
+ exsolutions from fusion and solidification: chalcopyrite, magnetite, iron/nickel alloy crystals, Fe-Ni spherules with a mixture of schreibersite and troilite.
And also devitrified sections (cristobalite).
Composition of moldavites: SiO2 (≤80% + K2O, CaO, MgO, Al2O3, Na2O...).

As for the Libyan glass, it is an exsuda (personal neologism) made of pure silica (> 98 %), extracted from the rocks of the North African savannah by the action of the H2O reagent in the Dantean conditions of heat and pressure caused by the nozzle of a comet in flames (before impact). Silanol (Si - OH) functions and H2O molecules destroyed the silicate networks to form future aggregates of pure silica gel.
The extraterrestrial signature is given by black dust trails from the comet's nucleus, with a particular iridium content of 150 ppm.

I summarized the famous Tunguska event (1908) in an article for my club (AGAB-Liege). The horizontal flight of this burning fireball (over more than 1200 km, with partial explosions, with large thunderstorms) was observed by thousands of people.
A comet cannot have a crust because the ice sublimes into hydrogen and oxygen. A comet is a poor conductor of heat. It never melts. No fusion crust forms. The water ice sublimes into a mixture of hydrogen and oxygen which burns with an explosion (thunderstorms).
A meteorite is red-hot, without flames, producing a small crust of superficial fusion, the internal part remaining at around -120°C.
This Tunguska astrobleme was studied in great detail by Leonid Kulik in the late 1920s. The comet exploded at the epicenter at an altitude of 5 to 10 km, laying the branchless trunks of conifers aligned radially around the epicenter, a zone that has become swampy.

The comet exploded (chemically) without forming a crater. However, there was a final mechanical explosion that ejected a large piece of the comet (ice), which dug out Lake Ceko. Then, the large part of the comet ignited like a rocket to escape the atmosphere.
The obliquity of this trajectory has even been calculated, probably caused by the principle of conservation of motion. The interpretation of the existence of the secondary crater (Lake Ceko) was proven by an Italian team at the beginning of this century, led by Gasperini (Bologna). These physicists carried out a very precise study of Lake Ceko, proving that it was the crater of the secondary ejecta. Kulik had not noticed this. In fact, this crater is not at the epicenter but about ten kilometers away, in the direction of the comet's path (conservation of motion).
(epicenter : approx. 50°7 - 101°9 – Google Earth)
But these brilliant researchers never uttered the word “comet”! We had no reference publication on paper! Hola Herwig.

Tunguska comet. Caption in Russian: Glimpse of the fireball over the Evenki (or Evenk) encampment.
That morning, two suns were shining in the sky. Museum of Lomonosov Moscow State University.
Lower caption difficult to read, including the name of the painter (N.I. Fidor??? – 1914/1990). © M. Zavasky.

[Craig Hagstrom]

Here's something fascinating. I wanted to do a better image of the cavities in my LDG_11 sample. It has what appears to be a seed cast with several leaf tips at its mouth, and this cast is visible below as a faint tunnel. The piece has some dents that are perhaps seed casts on top. The main cavity is huge, and I presumed it would be a group of seed casts. There is a cusp inside that I took to be the contact point between two seeds.

I've been working on this for about a month. I tried to get a laser scan but couldn't find the resources. So I did a photgrammetric scan of the outside, and made silicon casts of the cavities and then did photgrammetric objects from those. I'm in the process of stitching it together. The faint tunnel on the left side of the first image is partly stitched, and has assumed the same transparent color as the base piece. The large cavity is not yet joined and is here in gray. I thought a picture in this state would make it easy to see.

I expected to see something like seeds, but this looks like a lobe of something like a ground wasp nest. Notice the tiny tunnel at one end. The open side of the cavity would have been connecting to the rest of this chamber, not a clue how big it was. I don't want to get ahead of myself, but this looks like a burrow. Ants don't make structures, but ground wasps do make paper structures.

Finding seed casts was mildly surprising. Finding the remains of leaves still astonishes me. I have no idea what to do with a burrow. I realize that high temperature and pressure is the accepted story for LDG, but this is looking more and more like warm pudding. If anyone's BS detector pegs, I forgive them, but this is what I got.

Anyone know someone with a background in insect archeology?

[Craig Hagstrom]

Here's just the cavity.

[James Catmur]

I need to dig out the Iranian desert glass I found in 1979

[Craig Hagstrom]

I put a preliminary copy in STL format on Thingiverse, if anyone wants to mess with it. You can print a model of it, but you'll have to set that up. The model as is has no thickness and is not manifold.

http://www.thingiverse.com/thing:7008120
(link normalized by FMF)

[Craig Hagstrom]

I uploaded a model of the entire rock, same link as above. I stitched the cavity model into place in a model of the rock, both done with photogrammetry. Pretty pleased with the fidelity, but it doesn't have near the resolution of a laser scan. The most interesting feature of the cavity is the little vestibule at what appears to be the nest entrance. I recall long ago reading that some ants use gatekeepers, and it's tempting to see this in that light.

This model is manifold and 3D printable.

Don't have a clue how this thing was formed.

[Craig Hagstrom]

I've uploaded a new release of my report. This includes 3D images of the two specimens with significant voids - one with apparent seeds, one with the insect nest.

Had an epiphany with my morning coffee and added beta radiation to my list of possible LDG formative causes. Beta radiation fluidizes silica without melting. The insect nest goes from beyond belief to mildly interesting when beta radiation is the cause. It doesn't explain zircon and reidite, far as I know, so it's not a complete solution. Lots more to research.

https://www.academia.edu/128186762/Libyan_Desert_Glass_Structure

https://archive.org/download/glass-49/Glass_71.pdf
(links normalized by FMF)