168 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1964 



ture, and time in the formation of these patterns. A method of analy- 

 sis using the method of finite differences was developed for the 

 diffusion-controlled growth of the Widmanstatten patterns. As nec- 

 essary inputs to the growth analysis, the interdiffusion coefficients for 

 the Fe-Ni system, as a function of pressure, temperature, and compo- 

 sition, were measured, as were the diffusion coefficients in both the a 

 and y phases. The Fe-Ni phase diagram was also redetermined at 

 temperatures above 500° C Dr. Goldstein proposes two alternative 

 models for the origin of meteorites in which the Widmanstatten 

 pattern formed at low pressures. 



Dr. Matthias F. Comerford has initiated a program to investigate 

 defect structures in meteorites and micrometeorites. An attempt is 

 being made to relate the substructure observed in extraterrestrial ob- 

 jects to the thermomechanical procedures required to produce simi- 

 lar structures in laboratory alloys. The environmental effects of both 

 pressure and temperature upon the kinetics of nucleation and growth 

 of these defect structures can be examined in some detail. Prelimi- 

 nary results indicate that both effects are present and may act in 

 opposing ways. 



Dr. Fireman and his associates conduct a broad program of re- 

 search to measure cosmic-ray-produced radioactive and stable isotopes 

 in meteorites, in recovered satellites, in dust collections from the polar 

 regions, and in deep-sea sediments. In this program one must con- 

 stantly improve and maintain low-level comiting equipment and other 

 types of analytical apparatus. The group has determined the time 

 various meteorites were exposed to cosmic rays. The youngest is the 

 Farmington meteorite, which was exposed for only 10,000 years ; the 

 oldest stony meteorite is Norton County, exposed for about 400,000,000 

 years. Results on recovered satellites indicate that in addition to 

 cosmic rays there are isotope effects produced by Van Allen particles 

 and solar flares. These effects are quite different from cosmic-ray 

 effects. 



An important advance was made during the past year when James 

 C. DeFelice and Dr. Fireman obtained sufficient material to measure 

 the short-lived argon-37 in the whole-rock, magnetic, and nonmagnetic 

 phases of the recently fallen chondrite Peace River. Although the 

 radioactive contents are similar to those of other newly fallen chon- 

 drites, the ratio of argon-37 to argon-39 is somewhat lower than they 

 have previously observed. Also, its carbon-14 is lower. In another 

 analysis, the cosmic-ray exposure age of the Pribram meteorite was 

 found to be identical to the value of the exposure age obtained for the 

 Bruderheim fall, which is typical for chondrites. The tritium, argon- 



