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Roger Warin
Joined: 23 Jan 2013
Posts: 1207
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Posted: Jan 14, 2025 14:21 Post subject: Re: Libyan Desert Glass Structure |
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Hi,
I intend to present my ideas to you. Just a little more patience
It is still complex to describe the LDG in a few words.
At least the photos will surprise you.
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Roger Warin
Joined: 23 Jan 2013
Posts: 1207
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Posted: Jan 14, 2025 14:47 Post subject: Re: Libyan Desert Glass Structure |
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Hi LDG lovers - FMF LDG
At the risk of sounding like an iconoclast, I'm a chemist who became one because he loved minerals. My experience as a researcher-teacher can tell you that I'm not afraid of synthesizing molecules on a bench, nor of spectrometers, since I ended my career specializing in NMR (liquids and solids-MAS).
But I do like simple ideas, and that's when I get things wrong.
I've been collecting thin sections of meteorites since the '90s. Being very close to Zelimir Gabelica, I visited all the shows in Ensisheim (France).
So it's been a long time since I've held this famous desert item in my fingers.
I've given it a lot of thought, and at first I was wrong to subscribe to Norbert Brugge's (Germany) theories on the terrestrial origin of Libyan glass. His arguments seemed very plausible and respectable, especially as no coherent thesis had been published.
The crux of the matter for me, as a chemist and amateur geologist, was that silica glass is 98-99% SiO2.
I really understood the problem when Marcos Campo Venuti gave me the answer on the origin of Pele's tears, on this Jordi forum.
So here's my point of view, and I'm sorry if I come across as an iconoclast.
I only ask a question that I believe to be original, like some of my photos (taken in January 2013) that remain unpublished.
And of course the discussion is open.
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Roger Warin
Joined: 23 Jan 2013
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Posted: Jan 14, 2025 14:50 Post subject: Re: Libyan Desert Glass Structure |
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LDG STORY
Around 26 million years ago, a cataclysm occurred in Egypt, in a place that today belongs to the Libyan Desert. The presence of Libyan Desert Glass (LDG) bears witness to this phenomenon. This LDG glass was used as a tool during the Pleistocene.
This glassy phase can be gem-quality, but this is very rare. The photo shows a strictly transparent specimen containing cristobalite inclusions.
These photos actually show crystalline spherules, not single crystals. They were formed in an almost pure silica gel.
The origin of this gel explains the cause of the phenomenon.
In response to my question, a director of the French CNRS told me that the fall of a celestial body could have occurred on a mass of diatoms (whose shell is made of almost pure silica).
Objection : the probability of such an event occurring is very low.
In fact, there is a similarity (without witnesses) with the Tunguska blast (Siberia - 1908) caused by the partial fall and high-altitude explosion of a comet.
LDG glass is not impactite or tectite, but an “exudate” of a terrestrial rock, a sandstone rock from this African territory, which was not a sandy desert, but a kind of savannah. These silicate rocks have lost SiO2, which in these Dantean conditions is soluble in water (yielding numerous complex orthosilicic and polysilicic acids – also tectosilicates)). These silica gels are currently destined for major industrial applications.
Mineral: | Libyan Desert Glass |
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Roger Warin
Joined: 23 Jan 2013
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Posted: Jan 14, 2025 14:55 Post subject: Re: Libyan Desert Glass Structure |
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The LDG precursor gel obviously contains traces of the signature of celestial bodies (astrophysicists' famous dark dust). This dark matter is comparable to the very primitive cold carbonaceous chondrites of the Ivuna (CI) or Mighei (CM2) type.
These space-origin impurities include compounds similar to those found in CAI's and, as the best signature, space-origin iridium (~ 150 ppm Ir) (crustal abundance of Ir on Earth = 0.001 ppm - the lowest value of all natural elements).
Striations in LDG (Schlieren) are correlated with LDG viscosity.
Silica gel (not dehydrated) is an amorphous phase with siloxane bridges created by condensation of two intermediate silanol groups with the loss of one water molecule.
In fact, this amorphous “tectosilicate” phase sometimes gives up crystalline anhydrous SiO2 in the structure of high-temperature, high-pressure amorphous phases, such as cristobalite (β- Cristobalite is a material that is NOT an approved mineral species (IMA). Also Tridymite, Lechatelierite (amorphous) in these high temperature conditions between 870 and 1470°C.
Mineral: | Cristobalite |
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Roger Warin
Joined: 23 Jan 2013
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Posted: Jan 14, 2025 14:58 Post subject: Re: Libyan Desert Glass Structure |
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I was surprised when I found on my photo the Maltese cross that I had already shown you in my photos of luxulianite (UK).
Indeed, spherulites have particular optical properties. Thus, observed under an optical microscope between a crossed polarizer and analyzer, they often present an extinction in the shape of a Maltese cross, along the axes of the polarizer and the analyzer.
The optical properties of spherulites result from the dielectric nature of the phases, the anisotropic character of the crystals, and the particular organization of these crystals within the spherulite. This texture induces an extinction in the shape of a Maltese cross whose position is constant and independent of the rotation of the microscope stage.
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Roger Warin
Joined: 23 Jan 2013
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Posted: Jan 14, 2025 15:00 Post subject: Re: Libyan Desert Glass Structure |
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Note that cristobalite spherulites with semi-crystalline phases are similar to those observed in crystallography of (bio)polymers.
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Roger Warin
Joined: 23 Jan 2013
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Posted: Jan 14, 2025 15:01 Post subject: Re: Libyan Desert Glass Structure |
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OTHER CURIOSITIES: - SCHLIEREN – BUBBLES - SPACE MATERIAL
As for Schlieren, they result from an optical phenomenon highlighting the local contractions and decompressions of the gel in its flow which does not seem to have been turbulent during a slow cooling.
This German term has become a general characteristic of glasses; it means "striations". These Schlieren materialize the lines of flux by deflecting the light because of the existence of a gradient of refractive indices. These differences in index are correlated with the gradient of the flux density. This pattern of curved shadows follows the regions of low density (expansion) and high density (compression).
Other inclusions are numerous to the point that truly clear specimens are very rare. Chains of microbubbles or bubbles, particularly of cristobalite, are also found. Bubbles are small cavities created during the dehydration of silica gel.
There are also chains of microbubbles or bubbles. Bubbles are small cavities created during the dehydration of silica gel.
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Roger Warin
Joined: 23 Jan 2013
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Posted: Jan 14, 2025 15:03 Post subject: Re: Libyan Desert Glass Structure |
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Some bubbles and Sclieren in LDG. © R. Warin.
BLACK VEILS :
This phase presents traces of siderophile elements including iridium, osmium, platinoids, which form black veils during drying.
In fact, we find space dust there, many of which are found in primitive chondrites.
This means that the minerals found in the CAI’s are also found in Libyan glass. The origin of these compounds is truly spatial.
This phase presents traces of siderophile elements including iridium, osmium, platinoids, which appear during the drying of the gel.
The black bands contain carbon (the black dust of astrophysicists), traces of siderophile elements such as Co, Ni, Ir, Re, Os which are attributed by most authors including Koeberl as being traces proving the fall of a chondrite. Koeberl did, however, recognize compositional differences between the surrounding desert sands and the Libyan glasses. In fact, I am more likely to think of a cometary nucleus or the material that fouls comet ice. Incidentally, the ice is in a special polymorphic phase at -220°C.
Analysis of the dark streaks in the LDG reveals high abundances of Al, Ti, Mn, Cr, Fe, and Ni, as well as a pronounced correlation between the abundances of Cr, Mn, Fe, and Ni. Since the Fe/Ni, Mn/Ni, and Cr/Ni ratios are all clearly nonchondritic, the source of this material is most likely extraterrestrial, a comet.
CONCLUSION :
The existence of cristobalite in LDG glass proves that this silica phase underwent significant heating, of the order of 1800 °C. This argument simply specifies the temperatures that were reached during the extraction of pure silica SiO2 that formed the silica gel that became LDG.
It would seem that I am the only one to compare this origin of LDG to the partial fall of the Tunguska Bolide (explosion of a comet at an altitude of 5-10 km and fall of a solid debris from this comet that dug Lake Ceko – June 30, 1908). Small cavities, without craters, were found at the epicenter, with iridium, of course.
In Egypt, it was the explosion of another comet that provided the solvent needed to extract SiO2 from local silicate rocks.
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marco campos-venuti
Joined: 09 Apr 2014
Posts: 213
Location: Sevilla
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Posted: Jan 14, 2025 17:06 Post subject: Re: Libyan Desert Glass Structure |
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Hi Roger, beautiful photos and interesting discussion. I am always happy to discuss with you. I would like to clarify some points on which I have studied for a long time especially on the origin of spherulites. I am a volcanologist and the problem of natural glasses has always touched me closely.
1- Who did you get the term "exudate" from? It sounds similar to what I had defined as partial melting. However, the term exudate in geology has another meaning, so it does not seem appropriate in this context. The concept is that during the explosion of the comet the large amount of water allows the partial melting of a highly silicate component. Indeed, this fraction exudes from the rocks. In fact, to have a complete melting it lacks the time. If I completely melt the mantle I have a peridotite, with a partial melting I have a basalt, but with a very limited partial melting I can also have a rhyolite. There are rhyolites associated with ocean ridges. It is not necessary to invoke a siliceous substrate (diatomeite) because any rock can be ok to extract the LDG by means of partial melting.
2- Silica gel is not stable at temperatures above 300 C. What you call gel I believe is a high temperature melt. It is not magma, it is not lava, but it is certainly not a gel. In the models that seemed more plausible to me I had read about sublimation, so it would correctly be a vapor, the result of sublimation. When the steam condenses then we have glass. For this reason we are not in a context of high pressure, but only high temperature. Because we are in open space. High pressure minerals are found at the bottom of a crater, but not in a melt that forms in air.
3- I do not see analogies with Tunguska because in that event there is no glass or any other fragment of extraterrestrial origin. So even if it were a comet, and I am not sure because we have never found the smoking gun, it would be a small comet that was unable to form glass. So your analogy is a bit risky.
4- Where I disagree with you and many scholars is on the origin of spherulites, the subject of a work of mine in preparation, but based on a hypothesis already proposed many years ago.
Spherulites are the main theme of many misinterpretations in geology and especially in the study of obsidians and the genesis of thunder eggs. Spherulites form at low temperatures and are not due to crystallization in a fluid. In a silicate magma if something crystallizes phenocrysts of a single mineral are formed, not polycrystalline spherules that include different minerals. Furthermore, the composition of spherulites includes amorphous silica and cristobalite, two minerals that are incompatible with each other and both incompatible with a rhyolitic magma. They form at very different temperatures, one at room temperature and the other at >1470 C. Cristobalite is nothing more than opal as is that of snowflake obsidian. Finally we saw that obsidians are aphyric precisely because they have a Tg that is too high and do not have time to crystallize. Some authors believe that crystallization occurs below the Tg but at temperatures maintained high for years. Obsidian remains fluid for minutes, not years. Even in tektites and in LDG that I consider a tektite, cooling is very rapid. We are in a drop of glass that is falling. No fragment has been found with evidence of deformations from impact with the ground so they fell already cold. Basaltic volcanic bombs have the shape of cow dung because they fall still molten. Schlieren or fluid structures must form very quickly. As well as the few bubbles that indicate an albeit brief degassing.
Below the Tg, obsidian is already solid and therefore cannot crystallize. What is the difference between a high temperature spherulite (HTCD, high temperature crystallization domains by some authors) and a phenocryst? Why should we have high nucleation at high temperature? The HTCD model simply does not work. All experimental works on spherulites shows pictures of groups of crystals aggregated around a nucleus, therefore phenocryst aggregates, but not microcrystalline spherical structures and therefore not spherulites.
Spherulites form in any ancient glass and are well known in the glass of the Murano dump and in many medieval stained glass windows. Any archaeologist is familiar with these structures and they call it pitting. The Alkali Silica Reaction dissolves glass at room temperature and transforms it into a hydrated silica gel (this is a gel). This reaction starts from a point, often an air bubble or a small crystal of feldspar (alkalis help the reaction because at pH>9 the solubility of silica becomes very high) and creates a spherical cavity. Obsidian is porous for water which has a small molecule, but the molecules of silica polymers are large and cannot escape. So the water enters, but the silica gel remains trapped. However if the ASR is very intense we can produce silica monomers which are molecules small enough to escape. This gel then recrystallizes inside a spherical space at room temperature and various poorly crystalline minerals are formed.
Your beautiful photos show spherulites with obvious desiccation cracks. The supposed Maltese cross honestly does not convince me. They have a septarian type crack model so they indicate a volume decreasing. In fact the ASR is a reaction that consumes water and the result is a hydrated gel. When this gel dries, cracks form.
will continue ...
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