General: The mesosiderites are named for the Greek words mesos for “middle” or “half”, and sideros for “iron”, meaning “half iron”. In fact, they are typical stony-iron meteorites, most consisting of approximately equal portions of nickel-iron metal and silicates.
Description & Mineralogy: Texturally, mesosiderites are a complex mixture of a nickel-iron metal portion and a heavily brecciated silicate portion, consisting of mostly pyroxene and plagioclase. Strangely, the silicates are obviously evolved igneous rocks, representing the crust of an achondritic parent body. They are quite similar to eucrites, diogenites, and other members of the HED group, even plotting on the same oxygen isotope fractionation line. However, the metal in mesosiderites is similar to group IIIAB irons, obviously representing the core of a distinct, differentiated asteroid, genetically unrelated to the precursor of the eucritic and diogenitic portion.
Formation history & Origin: This suggests a complex formation history for the mesosiderites and their parent body. One theory has them formed by the collision of two differentiated asteroids, allowing the still liquid core of one asteroid to mix with the solidified crust of the other. This scenario includes the collisional disruption and gravitational reassembly of at least one of the asteroids – the one that later became the parent body of the mesosiderites. It is still heavily debated whether the HED parent body, 4 Vesta, actually represents one of these asteroids.
Members: Excluding all probable pairings, the mesosiderite group comprises about 58 distinct members, while seven members represent witnessed falls. Famous members of the mesosiderite group are the witnessed falls of Estherville, Iowa, USA, in 1879, and Lowicz, Poland, in 1935. Another renowned member is Vaca Muerta, a find from Chile. Several hundred individuals of this well-preserved mesosiderite have been recovered from its strewn field in the Atacama Desert, making it the most common mesosiderite in private and public collections.