Primitive FeNi metal grains in CH carbonaceous chondrites formed by condensation from a gas of solar composition
Some FeNi metal grains, similar to 150 mu m in apparent diameter, in CH carbonaceous chondrites are concentrically zoned in Ni (similar to 5-10 wt%), Co (0.2-0.4 wt%), and Cr (0.3-0.8 wt%); Silicon is present at the similar to 0.1 wt% level. These observations are consistent with predicted gas-solid condensation from a gas of solar composition at temperatures of similar to 1370-1270 K and total pressure of similar to 10(-4) bar. Estimates of FeNi metal grain growth and cooling rates in this temperature range are consistent with brief and localized thermal episodes in the solar nebula. Compositionally similar FeNi metal grains have also been reported in CR and Bencubbin-like chondrites. Because FeNi metal is highly susceptible to secondary alteration (i.e., metamorphism, melting, oxidation), the observed FeNi metal condensates in CH, Bencubbin-like, and CR chondrites indicate that these meteorites experienced no thermal processing after their lithification and thus are among the most primitive meteorites in our collections.