Canadian Rockhound: The HBO Meteorite

The HBO Meteorite, the largest single meteorite known in the world today, is on the HBO farm situated 25-km northeast of Otavi and near Grootfontein, in Namibia. When a meteorite enters the earth's atmosphere at an extremely high speed, the friction is so great that the meteorite burns up in the atmosphere and in the process produces a bright streak of light, which is readily visible at night and is commonly called a shooting star. A huge belt of such meteoritic material, the so-called asteroid belt, is present in the space between the planets Mars and Jupiter. Scientists have not yet reached agreement about the origin of these meteorites. It can, however, be stated that whatever falls into the earth from space is part of our solar system that developed some 4,600 billion years ago.

The HBO Meteorite,near Grootfontein, Namibia.
Photo by Margret Grathwohl.

There are two main types of meteorites: iron meteorites and stony meteorites. Namibia is world-famous for its meteorites. The most extensive meteorite shower known is found in the southern part of the country and is called the Gibeon Meteorite Shower. It is estimated to have occurred over an area of 20,000 km. The more concentrated center of the shower extended over an area approximately 2,500 km in the vicinity of the village of Gibbon. The Hoba Meteorite was first described by Johannes Hermanus Brits in 1920 and was declared a national monument on 15 March 1955 with the permission of the landowner at the time, farmer Mr. O. Scheel. The meteorite weighs approximately 60 tonnes and measures 1.95 by 2.84 meters. Its thickness varies between 122 and 75 cm. The shallow pits and depression on the horizontal upper surface of the meteorite were caused by corrosion during its passage through the earth's atmosphere. Minor oxidation has occurred on the surface since. Due to the presence of a rare radioactive nickel isotope with a half-life of less than 80,000 years, scientists were able to determine that the Hoba Meteorite fell to earth less than 80,000 years ago. Analyses of age of the meteorite vary between 190 and 410 years.

The Hoba Meteorite consists of 82.4% iron, 16.4% nickel and 0.76% cobalt. Other elements present are traces of carbon, sulfur, and chromium, copper, zinc, gallium, germanium and iridium. Scientifically it is termed an ataxite, a meteorite with high nickel content. Under the microscope, material from the Hoba Meteorite displays the typical compact ataxitic structure with faint lines wedges and patches. The main minerals are kamacite (a nickel-iron alloy with 5-7% nickel) and taenite (a nickel-iron alloy with up to 65% nickel). High magnification shows intergrowth of the kamacite and taenite needles in the typical Wilmannstaetten structure. The meteorite also contains the more rare meteorite minerals schreibersite {(FeNi)₃P}, toilite (FeS) and daubreelite {FeCr₂S₄}.

The Hoba Meteorite is situated on the edge of the Kalahari plain that extends to the east and southeast. This plain is underlain by white calcrete of the Kalahari Group that fills the valley floors in the area. Underlying the calcrete are ancient granites as well as dolomites and limestones of the Otavi Group which also making up the surrounding hills. No crater or altered rocks have been found associated with the impact site. After the meteorite fell, a layer of calcrete gradually covered it. This calcrete was formed by the evaporation of near-surface groundwater, which carried calcium carbonate derived from the surrounding Otavi limestone. Today the region receives a maximum annual rainfall of only 175 mm and near the surface groundwater is less than abundant. The calcrete therefore suggests a more humid climate in the recent geological past.

Unfortunately vandals have damaged the meteorite. In 1985, Roessing Uranium Ltd. made funds available to the National Monuments Council to combat vandalism. In collaboration with Roessing, the council launched a project to protect the meteorite and make the surroundings more attractive for visitors. Mr. J. Angelbrecht, the landowner since 1987, donated an area for the development of the site. Subsequently, an information center was established to meet educational needs. The facilities were opened on 31 July 1987.

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