518 



Annals New York Academy of Sciences 



the carbonaceous chondrites have a variable, and frequently lower, content of 

 Na and K than the ordinary chondrites. In the most extreme case, Nogoya, 

 this depletion amounts to a factor of ^^4. Urey, therefore, suggested that the 

 carbonaceous chondrites were derived from the ordinary chondrites [specif- 



0.8- 



FiGURE 2. Stability relations among some of the important constituents of carbonaceous 

 chondrites, as a function of reduction potential and hydrogen ion concentration. Solid lines 

 show boundaries between solids and aqueous species at an activity of the latter of 10"" m; 

 dashed boundaries, those between aqueous species at 1:1 ratios. Temperature = 298° K.; 

 total pressure = 1 atmos. Total activity of dissolved sulfur species = 0.1; of carbonate 

 species, 0.01. Most of the constituents of carl)onaceous chondrites could coexist under equi- 

 librium conditions at Eh -^ —0.2 and pH 6 to 10. The exceptions are FeS (in place of which 

 FeSo would be expected) and (Mg,Fe)CO:! . The absence of FeS.> was discussed in the text. 

 The presence of (Mg,Fe)CO:j is not surprising: although i)ure FeCOn is unstable under the 

 particular conditions indicated, magnesium-rich breunnerite is likely to be stable. Also, an 

 increase in the total carbonate, and a decrease in the total sulfur activity will make FeCOs 

 stable in the triangular field bounded bv the dotted line. This figure has been adapted from 

 Garrets (1960), figures 6.11, 6.18, 6.19,' 6.20, and 6.21. (Rejjroduced from DuFresne and 

 Anders, 1962a, with permission of the editor.) 



ically, the high iron group, Fe/Si ^ 0.85, Urey and Craig (1953)], by an altera- 

 tion process that depleted the alkalis while introducing S, C, and a few other 

 elements in free or combined form. 



This picture has become less satisfactory now that the abundances of various 

 trace elements in meteorites have been determined. Most elements occur in 

 meteorites in approximately their "cosmic" abundances, as given by the semi- 



