Description

The meteorite weighs approximately 60 tons and measures 2.95 by 2.84 meters with a thickness between 1.22 and 0.75 meters. The shallow pits and depressions on the horizontal upper surface of the meteorite are the result of melting of the outer surface during the passage through the Earth’s atmosphere. Minor oxidation has subsequently occurred on the surface, and is responsible for the green stains of secondary nickel minerals present on the surface of the meteorite. The meteorite is separated from the surrounding calcrete by a 30 cm thick layer of magnetic, dark-brown iron shale composed largely of limonite, with minor magnetite and trevorite [NiFe₂O₄]. This layer contains iron, nickel and cobalt in the same proportions as the meteorite itself, indicating that the oxidation proceeded in situ without any leaching away of components. Before oxidation, the meteorite could have weighed about 88 tons (Marvin, 1999).

The Hoba Meteorite consists of 82.4% iron, 16.4% nickel and 0.76% cobalt. Other elements present in traces are carbon, sulphur, chromium, copper, zinc, gallium, germanium and iridium. The Hoba Meteorite is therefore classified as an ataxite. 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). Intergrowths of kamacite and taenite needles form the typical Widmannstätten structure (8.12.2). The Hoba Meteorite also contains the rarer meteoritic minerals schreibersite [(FeNi)P3], troilite[[FeS] and daubreelite [FeCr₂S₄]. McCorkell et al. (1968) measured a significantly high content of a rare, radioactive nickel isotope, and assumed that this represented less than one half life of the isotope, suggesting that the Hoba Meteorite fell to Earth less than 80 000 years ago.

Brief History

The Hoba Meteorite is situated 20 km W of Grootfontein and represents the largest single meteorite known in the world today (Fig. 8.12.1). It was found by Jacobus Hermanus Brits in 1920 and was declared a National Monument on 15 March 1955. When first found, only a small portion of the meteorite was visible on surface. Early ideas of recovering nickel from the Hoba Meteorite were soon abandoned, since it did not prove to be economic. Some samples were removed using an oxyacetylene torch, leaving a long scar on one side, and others were taken from some drill holes. In 1954, a curator of the American Museum of natural History in New York attempted to purchase the meteorite to display it together with other large specimens in the USA. However, the railway line, which passes within 7 km of the Hoba Meteorite only, had an axle loading capacity of 24 tons only, hardly enough to transport a 60 ton meteorite! Another major obstacle was the transport from its location to the railway line. Through this good fortune (albeit unlucky for the curator), the meteorite remained in Namibia and is now one of its most prominent natural national monuments. Over the years, people continued to try to cut off samples from the meteorite, and in 1985, Rössing Uranium Ltd made funds available to the National Monuments Council to establish an information center and to protect the meteorite from further vandalism. The Hoba Meteorite is situated on the northwestern edge of the Kalahari plain, underlain by calcrete of the Kalahari Group. Underlying the calcrete are Palaeoproterozoic granites of the Grootfontein Complex, as well as dolomites and limestones of the Otavi Group, Damara Sequence. Surprisingly, no crater or altered rocks have been found associated with the impact site and the survival of the meteorite as a single mass suggests that it entered the atmosphere on a long trajectory at a sufficiently low velocity to allow a soft landing. After the meteorite fell, it was gradually covered by layers of calcrete. This calcrete was formed by evaporation of near-surface groundwater, which contained calcium carbonate derived from the surrounding Otavi Group limestone. Today, the region receives a maximum annual rainfall of only 750 mm, and near-surface groundwaters are less abundant. The presence of calcrete therefore suggests a more humid climate in the recent geological past.