Microstructural analysis of three iron meteorites : Arispe, Inland Forts and Ocotillo

The microstructure of three iron meteorites, Arispe and Ocotillo (coarse octahedrites), and Inland Forts (an ataxite), were examined using optical and scanning electron microscopy. Emphasis was placed on the plessitic structures in the two octahedrites; pearlitic, spheroidal, acicular, martensitic and comb plessite structures were common. The pearlitic and spheroidal plessite structures in Arispe were typically found in the vicinity of cohenite ((Fe,Ni) ₃C) ‘nests' . The pearlitic plessite often coarsened in these regions as well. The acicular and comb plessite usually were present in the bulk of the meteorite. Ocotillo contained numerous retained taenite stringers, and these stringers had decomposed to a variety of plessite morphologies. Regions of coarsening and spheroidization of the pearlitic structure were commonly observed. A large troilite-graphite inclusion also was present. The structures observed in the ataxite Inland Forts were different from those present in the two octahedrites. The microstructure consisted of small particles of ferrite with associated schreibersite, in a banded matrix structure. The matrix appeared banded due to an orientation effect of the structure. The matrix consisted of three regions, a coarse, oriented two-phase region, a fine, oriented two-phase region, and a clear region. Possible formation mechanisms of the plessitic structures are discussed, however all aspects of these plessitic structures are not fully explained.