What is a stony-iron meteorite?
A stony-iron meteorite is made of roughly equal amounts of iron-nickel metal and silicate rock. The two main kinds are pallasites and mesosiderites.
Stony-irons sit between the two better-known families: part rock, part metal. They are scarce, making up only about 1 per cent of meteorites seen to fall, and they are among the most admired specimens in any collection.
Pallasites contain crystals of olivine, a green to golden silicate mineral, embedded in a continuous network of iron-nickel metal. When a pallasite is sliced thin and polished, light shines through the olivine like stained glass. Well-known examples include Esquel from Argentina, Imilac from Chile's Atacama Desert and Fukang, a large mass found in China in 2000. Pallasites have long been thought to come from the boundary between an asteroid's metal core and its rocky mantle, though some researchers suggest that impacts mixed molten metal with mantle rock.
Mesosiderites look very different. They are breccias, rocks made of broken fragments, in which chunks of basalt-like and pyroxene-rich rock are jumbled together with metal. Their silicate parts resemble the crust of a differentiated asteroid, and the metal appears to have been mixed in during a violent collision. Vaca Muerta from Chile and Estherville, which fell in Iowa in 1879, are classic mesosiderites.
Because metal and stone behave so differently, stony-irons can be tricky to look after. The metal can rust if stored in damp air, and olivine crystals may loosen or crack as the surrounding metal corrodes, so collectors keep them dry. They can also be hard to recognise in the field: a weathered stony-iron may look like a rusty lump of rock until it is cut. Anyone who finds a suspicious heavy stone should leave testing to a specialist rather than cutting or applying acid themselves.