Pallasites are mixed meteorites in which olivine crystals occur suspended in metal. It is thought that this may be a sample of the layer at the interface between large asteroid cores and mantles. The same types of rock probably occur at this same interface on earth.
Indeed, in the case of an asteroid at least a few hundred kilometres in size, the quantity of radioactive elements initially present would have been enough to heat it strongly, causing partial melting and the formation of a ferrous core at its centre and a mantle around generating magmatic and volcanic processes.
During violent collisions, some asteroids were pulverised by the impact and this may well be the origin of metallic and mixed meteorites, depending on whether they are composed of an iron-nickel alloy, like siderites, or a mixture of the latter with other minerals, like pallasites.