Mesosiderite
A stony-iron breccia made of iron-nickel metal mixed with broken basaltic and pyroxene-rich rock.
Mesosiderites are the messy cousins of pallasites. Their name means "half iron", and they contain roughly equal amounts of iron-nickel metal and silicate rock, but the silicates are not olivine crystals. Instead they are fragments of basalt, gabbro and pyroxene-rich rock that closely resemble the eucrites and diogenites of the HED family, jumbled together with metal in a breccia.
How metal from a core became mixed with crustal rock is still debated. A popular idea is a violent collision between a differentiated asteroid and a body carrying molten metal, followed by very slow cooling, which is recorded in the metal's structure. The best-known examples include Vaca Muerta, found in the Atacama Desert of Chile, and Estherville, which fell in Iowa in 1879 as a shower of metal-rich stones.
On a cut and polished face, a mesosiderite shows bright metal grains and patches scattered irregularly through dark grey and greenish rock, unlike the tidy crystals of a pallasite. It is heavy and strongly magnetic. Because the metal rusts, many finds are stained brown. Keep specimens dry, ideally with a desiccant, and remember that finds belong to landowners or the state in many countries.
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