Mesosiderites make up one of the two groups of stony-iron meteorites. Unlike the pallasites, the mesosiderites appear to consist of impact-generated silicate-metal breccias - they're mixtures of asteroidal core & mantle & crustal materials. The silicate-rich portion consists of olivine, pyroxene, and calcic plagioclase feldspar, with a texture ranging from unmetamorphosed angular clasts to recrystallized silicates to melt matrix. The metal portion is iron-nickel alloy, having the minerals kamacite & taenite. The sample shown below also has a fair amount of troilite (FeS - iron monosulfide) in close association with the iron metal.
Genetically, meteoriticists have noticed that some of the silicates in mesosiderites have close chemical-mineralogical similarities to HED meteorites (rocks from the asteroid Vesta). The iron metal portion is chemically similar to group IIIAB iron meteorites.
So, what's the origin of mesosiderites? These rocks appear to be the result of impact mixing.
Impactor - an asteroidal iron core stripped of its crust and mantle
Impactee - Vesta or Vesta-like asteroid having basaltic-gabbroic crust
The rock shown below is from the Estherville Meteorite, a type A3/4 mesosiderite having a highly recrystallized silicate matrix. This meteorite fell to Earth at 5 PM on 10 May 1879 near the town of Estherville in northwestern Iowa, USA.
Published isotopic dates of Estherville silicates indicate crystallization ages from 4.422 to 4.556 billion years. The impact event has been dated to 4.4 billion years, and a significant reheating event occurred at 3.6 billion years.
Mesosiderite (above & below; 2.9 cm across) - cut & polished slice of the Estherville Meteorite. The blackish material is Fe-Ni metal. The rest of the rock is a mixture of pyroxene, plagioclase feldspar, olivine, and some troilite.
Mesosiderite - cut & polished slice (2.9 cm across), tilted to show silver reflectivity of Fe-Ni metal component & golden-brown color of the troilite (see labels below).
Mostly synthesized from Norton (1998) and Hutchinson (2004).