Analytical electron microscopy study of the plessite structure in the Canton iron meteorite

In order to gain a better understanding of the formation of plessite in iron meteorites, various electron optical techniques were employed to study the range of plessite structures observed in the Carlton fine octahedrite. Compositional and structural studies of twins in clear taenite and the cloudy zone were made. Transmission electron microscopy studies of martensitic and duplex a + y plessite regions show the presence of y-taenite rods, 10—200 nm wide, in an a-kamacite matrix. Scanning transmission electron microscope X-ray analyses showed Ni contents in the y rods of 43 wt°,’ and Ni contents in the a matrix of 3 wt Ni. The reaction path involves the decomposition of a2 martensite into a + y and these reactions occur below 200C and possibly below 100 C. Apparently the formation of plessite is intimately related to the formation of martensite and the further decomposition of martensite during the cooling history of the meteorite. It is quite probable that the martensite decomposition reaction has occurred in a large number of iron meteorites and is responsible for many of the observed plessite structures.