Origination stony-irons

Iron and stony-iron meteorites constitute —6% of meteorite falls (Grady, 2000). Despite their scarcity among falls, iron meteorites are our only samples of 75 of the 135 asteroids from which meteorites originate (Keil et ai, 1994 Scott, 1979 Meibom and Clark, 1999 see also Chapter 1.05), suggesting that both differentiated asteroids and the geologic processes that produced them were common. 

In this chapter, we have touched upon our current state of knowledge about iron and stony-iron meteorites, the processes that formed them and the places from which they originate. Our knowledge of all of these is far from complete. In this section, we briefly address the future of these fields. What questions remain unanswered Which are most pressing to understand the origin of iron meteorites What tools do we need to move forward ... 

The approximately 50 known classes of meteorite, excluding those known to have originated from the Moon or Mars, span a wide range of compositions. Most meteorites are stones. The remaining classes of meteorites, the irons and the stony-irons, are products of melting and differentiation. They consist mainly of metallic iron-nickel alloys, sulfides, carbides, phosphides, and igneous silicates. (See the chapter by M. Lipschutz in this volume). 

The largest class of meteorite finds is stony meteorites, made principally of stone. The general stony classification is divided into three subclasses called chondrites, carbonaceous chondrites and achondrites, and it is at this level of distinction at which we will stop. Before looking at their mineral and isotopic structure in more detail, it is useful to hold the composition of the Earth s crust in mind here for comparison. The Earth s crust is 49 per cent oxygen, 26 per cent silicon, 7.5 per cent aluminium, 4.7 per cent iron, 3.4 per cent calcium, 2.6 per cent sodium, 2.4 per cent potassium and 1.9 per cent magnesium, which must have formed from the common origin of the solar system. 

At the end of the eighteenth century, the average density of the Earth as a whole was already known to be twice that of the common cmst rocks. The original concept of an iron core was purely based on meteoritic observations. Stony meteorites provide the clue that iron is more abundant in accretion materials than in the Earth s mantle (Cox, 1989). A serious proposal of an iron core was not presented until the recognition of the two main classes of meteorites, stony and iron meteorites, in the mid-nineteenth century. The iron meteorites had been assumed to come from the core of a fragmented planet. The idea of an iron core was generally accepted once Oldham (1906) confirmed the presence of a core. 

Sen Gupta J.G. Abundances of the six platinum metals in some iron and stony meteorites Relationships to the theories of evolution of parent bodies of meteorites and the origin of these elements. Chemical Geology 1968 3 293-305. 

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