Microstructural and stereological aspects of the Gibeon Meteorite

Stereological and microstructural aspects of plessite pools in the Gibeon Meteorite have been studied. The cellular plessite pools were found to be considerably larger than the finger plessite pools. This difference in size has a significant effect on the ability of nickel to diffuse into the central regions of taenite, which will subsequently form plessite. The overall effect will be a decrease in the average nickel content in the larger cellular plessite pools due to larger diffusion distances. This fact most likely accounts for the highly tempered (spheroidized) appearance of the cellular plessite, because a lower nickel content corresponds to an elevated M-s temperature. Approximate nickel concentrations for the plessite pools were calculated, and were found to fall within the region on the phase diagram where the martensitic reaction is favored at temperatures above the monoeutectiod reaction. The results tend to indicate that a martensitic phase transformation occurs during the formation of the plessite pools in the Gibeon meteorite.