METEORITE BASICS: STONY IRONS
The stony irons are a mixture of stone matter with metal alloys. In this rather broad group, there are two vastly different types, called pallasites and mesosiderites. Although both are the stone/metal mixtures, their origins are totally different.
First, the pallasites are largely the silicate mineral olivine (called the gem stone peridot) mixed with Fe/Ni alloys. How do these pallasites form?
As in the iron meteorites, visualize the wholesale melting of an asteroid with the usual separation of metal to the core. Now imagine that the separation is not complete before cooling so that there are still some silicate minerals, such as olivine, remaining within the metal. This is the material that is to be the future pallasite. As in the irons, a shattering of the poorly separated asteroid provides some fragments which are pallasites along with others from the core which are irons. As a matter of fact, often the same meteorite is represented by pieces of normal pallasite character and by irons with very little or even no olivine.
The mesosiderites are also mixtures of stoney material with alloy metal. However, unlike the pallasites the stoney material is not olivine but is a wide variety of stoney compositions often in very irregular broken fragmental shapes. How do mesosiderites form? Again consider an asteroid in which a metal: stone separation has occurred after melting. At a much later time, a shattering of the asteroid occurs, making thousands of fragments which travel in space as a loose collection, much like a cloud of fragments. Gradually, these fragments attract each other by gravitational forces and so combine once again, but now as a random mixture of core metal and silicate mantle rather than as the original layered sequence in the original asteroid.
[left top] Imilac, a stony iron meteorite (pallasite) found in Chile's Atacama Desert, and [left bottom] Detail of the Estherville mesosiderite which fell in Iowa in 1879.