Case 6: Iron Meteorites and Mesosiderites

The upper half of the case displays iron meteorites. Although these meteorites constitute only about 4% of observed falls, they are readily recognized on the ground. The names of the different iron meteorite groups consist of Roman numerals followed by one or two letters. Iron meteorites are classified based on their bulk elemental compositions and are divided into two broad categories: magmatic irons and non-magmatic irons. Magmatic irons are formed by fractional crystallization in the molten cores of asteroids; non-magmatic irons are formed by impact melting of chondritic material followed by partial separation of molten metal from silicates.

On display are magmatic irons (Ainsworth IIAB, Gibeon IVA, Buenaventura IIIAB, Muonionalusta IVA, Cerro del Inca IIIF, and Sikhote-Alin IIAB) and non-magmatic irons (Anoka IAB, Morasko IAB, NWA 6931 IAB, and Watson 001 IIE).

The bottom half of the case displays some mesosiderites. These are stony-iron meteorites that consist of about half metallic iron-nickel and half silicate. The silicate portions consist of rocks related to the eucrites, diogenites and howardites. On display are cut faces of Vaca Muerta [found in Chile in 1861], Mincy [found in Missouri in 1857], Patwar [fell in India (now Bangladesh) in 1935], Clover Springs [found in Arizona in 1954], Veramin [fell in Iran in 1880] and Emery [found in South Dakota in 1962].

The Vaca Muerta mesosiderite contains multi-centimeter-size igneous pebbles, consisting of basalts, coarse-grained rocks called gabbros, and impact melts. Three of these pebbles are on display; also shown (outlined in black) is a large (12 cm) igneous pebble still enclosed in an individual specimen of Vaca Muerta.

Late-stage impact melting in the near-surface environment on the mesosiderite parent asteroid transformed fine-grained mesosiderites like Emery and Vaca Muerta to coarse-grained ones like Estherville [fell in Iowa in 1879]. The metal in Estherville has segregated into veins and rounded nodules.

For more-comprehensive definitions, please see the Glossary.