Fulgurite

  Fulgurites (from the Latin fulgur meaning thunderbolt) are natural hollow carrot-shaped glass tubes formed in quartzose sand or soil by lightning strikes. In the right kind of sand the extreme heat generated will form silica glass shapes that trace the path of the lightning. These structures are also sometimes referred to as petrified lightning. The glass formed is called lechatelierite which may also be formed by meteorite impact and volcanic explosions. As it is amorphous it is classified as a mineraloid.

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The tubes can be up to a couple of centimeters in diameter, and meters long. Their colour varies depending on the composition of the sand they formed in, ranging from black or tan to green or a translucent white. The interior is normally very smooth or lined with fine bubbles; the exterior is generally coated with rough sand particles. They are rootlike in appearance and often show branching or small holes. Fulgurites occasionally form as glazing on solid rocks (sometimes referred to as an exogenic fulgurite^[1]).

Fulgurites are a very rare phenomenon. A very large one was found in South Amboy, New Jersey. This was roughly nine feet long with a diameter of three inches (7.6 cm) near the surface of the ground, and tapered to roughly three sixteenths of an inch (5 mm) in diameter at the deepest point recovered. As is often the case due to the fragile nature of fulgurites, scientists were unable to extract it in one piece and the largest recovered fragment was a mere six inches (15.2 cm) long.

Fulgurites are notably found high on Mount Thielsen ("the lightning rod of the Cascade Range") where they form a brownish-green glaze on rocks (especially on the final five or ten feet of the summit pinnacle) and on the shores of the Great Lakes.

Possibly the finest fulgurite sample on display can be seen in Philadelphia, USA, at the Academy of Natural Sciences. It was discovered in 1940.

The largest fulgurite known is at the Peabody Museum of Natural History at Yale University, which has on display a 13-foot (4 m) long fulgurite from the shores of Lake Congamond in northern Connecticut. The fulgurite has been on display at the Museum since the 1950s, and is viewable in the new Hall of Minerals, Earth and Space as of May, 2006.

A specimen of fulgurite over 3 metres long is in the Natural History Museum in London. It is preserved in sections of over 50cm.

References

1. ^ Exogenic fulgurites from Elko County, Nevada: a new class of fulgurite associated with large soil-gravel fulgurite tubes (Rocks & Minerals, Sep/Oct 2004, Vol. 79, No. 5.)