524 



Annals New York Academy of Sciences 



The free energy change in the conversion of high-temperature minerals to char- 

 acteristic minerals cannot be calculated with any accuracy, because no thermo- 

 dynamic data exist for the latter or their terrestrial counterparts, the serpentine 

 and chlorite minerals. As a crude approximation, the following reaction may 

 be considered: 



MgoSiOj + H,0 (1) -> MgSiOs + Mg(OH)o 



for which AFoys is —20 kcal. per mole. This corresponds to about 0.1 kcal. 

 per gram of olivine, and because the products in this reaction are capable of 



rZ^ Wholly Molten (>I620°K) 



^ Zone of Fe-FeS Eutectic ( 1260° < T<I620°) 



H Zone of Liquid Water 



Figure 7. Temperature distribution in an internally heated asteroid, for a central tem- 

 perature of 1900° K. The location of the zone of liquid water is indicated. 



Table 4 

 Times for Water Loss from Asteroids 



reacting further to give hydrated siUcates, this value is probably conservative. 

 A chondrite of the type suggested as a possible precursor of the carbonaceous 

 chondrites, e.g., Ornans or Warrenton, contains more than 75 per cent olivine 

 on a normative basis. Thus, although possible contributions by the other 

 minerals are neglected, the average amount of free energy released in the forma- 

 tion of the characteristic minerals is likely to be close to 0.1 kcal. per gram. 



The extractablc organic matter in Orgueil comprises about 10 per cent of the 

 total carbon content (3.1 percent, Wiik, 1956). Thus, approximately 3 X 10"* 

 calories would be available for each gram of organic matter, assuming that none 



