Microstructural observations of spheroidization from a lamellar structure in iron meteorites

The Ocotillo and Arispe iron meteorites have a coarse Widmanstatten pattern of primary ferrite crystals, typical of many iron meteorites. These crystals are separated by thin sheets or plates of retained austenite, some of which have decomposed into various structures upon very slow extraterrestrial cooling and subsequent unknown thermal histories. One structure is a lamellar product, probably formed by a monotectoid reaction. Some regions in these two meteorites show that the lamellar structure has begun to change to a structure of spheroids of a nickel-rich phase in ferrite, and other regions have completely spheroidized. In some regions, the spheroidization process occurs by geometric instability of the lamellar plates. However, most of the spheroidization appears to be occurring at morphological defects in the lamellar structure (e.g., branches), or at the end of the plates of the lamellar structure. There is evidence that the processes are controlled by interphase surface diffusion.