fulgurite - set of natural glass tubes formed by lightning strikes on sand
Fulgurites form when a lightning bolt strikes the earth’s surface, fusing the material into a glass tube, sometimes with a hollow center. When lightning strikes sandy soil, the air and moisture in the sand are rapidly heated. Quartz sand melts at a temperature of roughly 1600˚C to 2000˚C depending on the moisture content. The name comes from the Latin fulgur, for lightning.
These were collected from a Sahara dune field in Morocco. Though the seller did not want to disclose the location, he did relate that these are found when an occasional strong regional wind moves the dune surface to expose the fulgurites. The collector then goes out with his ATV, drives around on the dunes and sifts the sand where these appear. In some years none are found at all.
These are Type 1 fulgurites, sand fulgurites, as classified by Matthew A. Pasek et al in Fulgurite morphology: a classification scheme and clues to formation, Contributions to Mineralogy and Petrology 164 (3): 477 - 492 . Type 1 fulgurites form in sand as tubular structures, sometimes with the central opening collapsed, and frequently coated with sand.
There is little lightning activity in the Sahara today, so the fulgurites found in the Sahara confirm that very different conditions existed in prehistoric times. Since fulgurites are glass, they are resistant to weathering and are usually well preserved for long periods of time.
Because fulgurites are amorphous, they are classified as a mineraloid.
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