We suggested in Section 3.3 that the two features that need to be explained in understanding chondrites are the formation of chondrules and the metal–silicate fractionation. Having understood, at least partially, these two processes, much of the remaining properties of chondrites will fall into place. We discussed chondrule formation in the previous chapter, now we discuss metal–silicate fractionation and how the chondrules and metal were assembled together to produce chondrites. This, we argue, amounts to discussing the origin of chondrites. Finally, we can examine the extent to which other chondrite properties relate to the ideas we have developed for these processes.

Chondrule sorting

We will consider several processes for sorting chondrules. First, we will consider the idea that size sorting is a primary property and a result of the formation mechanism. Second, we will discuss the idea that it is the passage of the chondrule through the local gas environment that sorts the chondrules. We will call this aerodynamic sorting. Next, we will discuss the idea that the sorting is a result of the process by which chondrules were made to move from their formation location to the location at which the present meteorite formed. We call this ballistic sorting. Finally, we suggest that abrasion could result in size sorting in some cases.

Primary processes

Most theories for sorting chondrules and metal grains are “secondary,” in the sense that they assume that a range of sizes previously existed and were later sorted.