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Fulgurites

Posted by Rock Currier  
avatar Fulgurites
September 28, 2009 07:55AM
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It seems that your initial question of value and marketability has evolved into a question of it's correct identification.

Alfredo, posed the first logical question or request to you, asking if you would image a cross-section for others on the thread to observe and comment on, you did not provide this. Next, I asked if your specimen displayed any "hollow" features, your response was no. You also added, "I do know that not all fulgurites are hollow." (RED FLAG). Beyond these questions and remarks, nothing has transpired to progress the most important question ie, Are these actually fulgurites?

The only way this can be sorted out is by examination and analysis. Do all sand fulgurites have a hollow channel? The answer is most do. In formation the ground is heated to an average of approximately 2500 C. The core of this strike as it enters the ground may be much hotter. The hollow channel that is formed represents the area which was essentially vaporized. Beyond this "hollow channel" concentrically, "melting" is initiated, with temperatures of somewhere between 1400 and 1800 C (at least) required the melt the sand. Upon cooling, a glass is formed. Beyond this concentic zone of melting, the surrounding soils, sands, etc. may be partially melted and as a consequence and upon cooling will form a mixed assemblage with phases of glass and minerals, that will be otherwise "attached" or "cemented" for lack of a better word to the greater fulgurite structure. Also "branches" can and are often are produced during this process, all having a similar cross-sectional profile, all this happening in mere microseconds.

You asked, "If the beach where these were found is made up mostly of broken down shells, to the point of sand, wouldn't the fulgurite be mostly calcium carbonate as well?" The answer is no. Calcium carbonate would be reduced, the heat would drive off the carbon dioxide and you would be left with an oxide of calcium, CaO, which could be chemically measured in the the fulgurite glass but again NO carbonate would exist at all. Fulgurite glass generally will have between 90 and 99% SiO2. Depending on the the initial composition of the groundmass other oxides, ie. Al2O3, Fe2O3, CaO, etc will exist as well.

Phil said, "Almost all manufactured glass contains calcium carbonate. The main ingredients for glass are: silica sand, sodium carbonate and calcium carbonate.(Soda-lime-silica glass). The calcium carbonate does melt and forms glass in the presence of silicates." This is not accurate. As I stated, when heated to temperatures necessary to form fulgurites, the CaCO3 (calcium carbonate) is reduced, driving off CO2 gas and producing CaO, calcium oxide, only typically a minor component in ANY glass.

Conclusions There is seldom a circumstance where you can postively ID a specimen as a fulgurite from an image, especially when NO cross-section is displayed. Michael should have a small portion of his specimen cut and polished, demonstrating a cross-section. This would give him an idea as to it's validity. If it then appears to be a fulgurite, it should be analyzed to determine if it is a "glass" rather than a mineral assemblage. Testing a small piece in muriatic acid could not hurt as well. If it is a calcium carbonate mineral assemblage, it will be highly reactive, fizz and completely dissolve. If it is a fulgurite it will not dissolve, with the only reactivity occuring as possible "attached" carbonates react. Additionally, I agree with Ralph concerning the specimens from "Joshua Tree".

Final Comments Phil, I think Joshua Tree Museum is wonderful with a great purpose, best of luck with it's future success. Michael, you really owe it to yourself to have this tested if you believe in it. Try contacting a the Geology Dept of a university in your area, most would be happy to help and you would be able to get the facts on it. Good luck Michael.

All the Best,
Ron Gyllenhammer



Click here to view Best Minerals F and here for Best Minerals A to Z and here for Fast Navigation of completed Best Minerals articles.

Rock Currier
Crystals not pistols.



Edited 2 time(s). Last edit at 09/08/2011 08:42PM by Uwe Kolitsch.
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