Description

The Gibeon Meteorite Shower is the most extensive meteorite shower known on Earth and covers a large elliptical area of some 275 by 100 kilometers centered on Brukkaros S of Mariental. Most fragments are located just southeast of Gibeon and to date, some 120 specimens with a weight of almost 25 tons have been recorded. Unfortunately, an unknown number have been collected unrecorded.

The meteorites are classified as octahedrites, the most common type of iron meteorite, and consist entirely of taenite and kamacite, two different crystalline varieties of an iron-nickel-alloy. Taenite (gamma-Fe with 8-55% Ni) and kamacite (alpha-Fe with 5.5% Ni) both have a cubic crystal lattice and form alternating bands orientated parallel to octahedral planes. This structure is known as Widmannstätten pattern, after its discoverer, Baron Alois von Widmannstätten, and is a characteristic feature of many meteorites (El Goresy, 1976). Besides iron, which is the main constituent, the Gibeon meteorites contain an average of 8% nickel, 0.5% cobalt, 0.04% phosphorus, small amounts of carbon, sulphur, chromium and copper, and traces of zinc, gallium, germanium and iridium. The Gibeon meteorite specimens range in size from one ton to just a few grams. A few rare crystals forming tetrahedrons and octahedrons have also been recovered, and probably resulted from the successive splitting off of thin kamacite lamellae along with the selective corrosion of intervening taenite layers (Marvin, 1999).

Today, the larger known Gibeon meteorites that have remained in Namibia, are displayed in the Post Street Mall and at the museum of the Geological Survey of Namibia in Windhoek, and a number of smaller ones form part of the reference collection of the Geological Survey of Namibia. While meteorites continue to be found in the area of Gibeon, it is very difficult to locate them in the field without a metal detector. Meteorites are protected by law in Namibia, and may not be removed from their original site. Despite this, a large number continues to leave the country illegally to be sold at international rock and mineral shows. Recently, the largest known specimen so far, weighing about one ton, was illegally removed and exported to the USA (Norton, 1998). This deprives the local people of an opportunity to develop a sustainable use out of this unique natural resource and national heritage, and also deprives the international scientific community of valuable research material.

Brief History

Small pieces of metal were collected by James Alexander in 1838 at a place “about three days journey NE of the mission at Bethany”. He described the pieces to be up to two feet square, and sent some material to the chemist John Herschel in London, who established the meteoritic origin of the material. However, it is well known that the local Nama people had been using the meteorites for a long time to produce spear points and other weapons. The first recorded large piece, weighing some 81 kg was carried by ox-wagon for 800 miles to Cape Town by John Gibbs before 1853 (Shepard, 1853) and from there it was sent to London, where the mineralogist to Queen Victoria, Professor John Tennant, purchased it. He forwarded it via New York to Professor Charles Shepard of Amherst College in Massachusetts, who studied the material in detail. At least ten pieces of Gibeon meteorites had been shipped to Europe by 1910 (Marvin, 1999), before Dr Paul Range, geologist of the German Colonial Administration, collected all the remaining meteorites he could find and mapped their occurrence between 1911 and 1913. The specimens were displayed in Windhoek, and a number of them were donated to various museums around the world. The Gibeon meteorites occur partially embedded in rocks of the Karoo Sequence and calcrete of the Kalahari Group. It has been calculated, from available evidence, that the Gibeon meteorites resulted from a meteorite body, measuring roughly 4 by 4 by 1.5 meters entering the Earth’s atmosphere along a northwesterly trajectory and at a low angle of 10° to 20° from the horizon. This body fragmented while still high in the atmosphere, so that the fragments themselves suffered thermal alteration by melting of the outer surface. This melting either resulted in smooth outer layers, or in molten material being pulled off in places by the drag of the atmosphere, leaving behind an uneven mass with deep, spherical cavities on the outer surface. These well developed thermal alteration structures prove that the fragments had an extended flight through the atmosphere before being deposited. The varieties of thermal- and shock-induced microstructures of the Gibeon meteorites is the greatest in the world and only matched by the Canyon Diablo Meteorite of northern Arizona (Marvin, 1999).