ON THE ORIGIN OF CARBONACEOUS CHONDRITES* 



Edward Anders 



Enrico Fermi Institute for Nuclear Studies, and Departments of Chemistry and Geophysical 

 Sciences, University of Chicago, Chicago, III. 



Carbonaceous chondrites are related to other classes of meteorites in many 

 ways, and much of what has been said about the origin of meteorites, in gen- 

 eral, appUes to carbonaceous chondrites as well. Like all other meteorites, 

 they are fragments of larger bodies. To reconstruct their history, we must 

 try to learn more about the nature of these bodies, that is, their size, number, 

 and location, and the chemical and physical processes that produced the de- 

 tailed structural and compositional features of the meteorites. 



Some of the principal hypotheses on the origin of meteorites are outlined 

 in TABLE 1. (A more complete review of the subject has been given by Anders 

 and Goles, 1961.) Each of these hypotheses can account for some 90 to 95 

 per cent of the properties of the meteorites, and it is only the last 5 to 10 per 

 cent that causes difficulties. There is just as much disagreement on the origin 

 of the carbonaceous chondrites (table 2). Mason (1960, 1961) and Ring- 

 wood (1961) assume that they represent some of the primitive material from 

 which the solar system formed; Urey (1961) believes that they are alteration 

 products of the high iron group chondrites, which are themselves several steps 

 removed from primitive material. Finally, Wood (1958, 1962) and others 

 believe that they are alteration products of a hypothetical, primitive chon- 

 drite, similar to Renazzo or Ornans (Fish et al., 1960; DuFresne and Anders, 

 1962a). 



Clues to the Origin of Carbonaceous Chondrites 



Mineralogy. Some clues to the origin of the carbonaceous chondrites can 

 be obtained from a study of their mineralogy. Results for 9 of these meteor- 

 ites are shown in table 3 (DuFresne and Anders, 1962a). The estimated 

 relative abundances are expressed as negative logarithms of 2; the entry 3, 

 for example, stands for 2"^ or 1/8. The minerals found can be divided into 

 three classes: conventional, "high-temperature" minerals; "characteristic" 

 minerals pecuhar to this class of meteorites; and trace minerals. In addition, 

 these meteorites also contain appreciable amounts of sulfur, hydrated MgS04,t 

 elemental carbon, and organic compounds. On the basis of their mineral 

 composition, the carbonaceous chondrites can be divided into 5 subclasses. 

 These show a fair degree of correspondence with Wiik's (1956) three classes, 

 established on the basis of chemical composition only. 



One can prove rather convincingly that the characteristic minerals are al- 

 teration products of the high-temperature minerals, rather than vice versa. 



* This work was supported in part by the U.S. Atomic Energy Commission. 



t The state of hydration varies with the temperature and the relative humidity at the 

 time of measurement. Very probably, the MgS04 was present as the anhydrous salt or as 

 the monohydrate at the time of fall, and became hydrated after exposure to atmospheric 

 moisture. Boato's (1954) measurements show that the water released below 180° C. has a 

 normal D/H ratio, and is probably of terrestrial origin. 



514 



