Gibeon Meteorite

An iron meteorite of the Octahedrite class, the most common class of the iron meteorite group (nickel iron meteorites, to be exact).

Origin of name: after Gibeon, a village in Namibia, about 160km north of Keetmanshoop and 65km south of Mariental.

Can be confused with: other iron meteorites

Localities: only in Namibia. The strewn field of the Gibeon meteorite measures about 120x390km and is one of the largest on earth.

Handling: the Gibeon meteorite contains approximately 92% iron and thus it can (and will) rust. We recommend the use of clear varnish before setting to protect the finished piece of jewellery from water and subsequent corrosion.

History: first mentioned by Captain James Edward Alexander in 1838. During an expedition into the heartland of (what today is) Namibia the British officer heard rumours of giant pieces of iron lying about. Alexander never reached the strewn field but could acquire some samples which he sent to London for analysis.

Worth knowing: when cut and etched with nitric acid iron meteorites show so-called Widmannstätten patterns.

^{Widmannstätten patterns on a disc of the Gibeon meteorite}

The reason for the formation of these patterns is the unequal resistance to acids of the two nickel-iron-minerals kamacite and taenite. Whilst nickle-poor kamacite is dissolved, nickle-rich taenite persists.

Iron and nickel form homogeneous alloys at temperatures below the melting point. These are called taenite. At temperatures below 900 to 600°C (depending on the Ni content), two alloys with different nickel content are stable: kamacite with lower Ni-content (5 to 15% Ni) and taenite with high Ni (up to 50%). Octahedrite meteorites have a nickel content intermediate between the norm for kamacite and taenite, this leads under slow cooling conditions to the precipitation of kamacite and growth of kamacite plates along certain crystallographic planes in the taenite crystal lattice.

The formation of Ni-poor kamacite proceeds by diffusion of Ni in the solid alloy at temperatures between 700 and 450°C, and can only take place during very slow cooling, about 100 to 10,000°C/Myr, with total cooling times of 10 Myr or less. This explains why this structure cannot be reproduced in the laboratory.

Chemical composition: 91,8 % iron; 7,7 % nickel; 0,5 % cobalt; 0,04 % phosphor; 1,97 ppm gallium; 0,111 ppm germanium; 2,4 ppm iridium