The microstructure of the Arispe meteorite

The microstructure of the Arispe iron meteorite, a coarse octahedrite with anomalous structure (IC-An), was studied using optical and scanning electron microscopy, with emphasis placed on the various morphologies of plessite seen in this meteorite: pearlitic, spheroidial, and martensitic structures, as well as microwidmanstatten plessite and comb plessite.

The pearlitic and spheroidal plessites were generally associated with each other and were found only in the vicinity of the large (~ 1 cm) cohenite ((Fe,Ni)₃C) "nests," while the microwidmanstatten and comb plessites were seldom seen in intimate contact with the cohenite.

Schreibersite and rhabdite (both (Fe.Ni)₃P) and Neumann bands were also observed. Rhabdite was present throughout the kamacite, although it was more prolific in some grains than others. Schreibersite occurred as both large (~ 1 cm long) inclusions and as small (~ 50 µm) inclusions in plessite and cohenite. The Neumann bands seen were either of a constant width, or were pinched and segmented (perhaps indicating reheating). They were seen to be very orientation sensitive, appearing very densely in some grains, and being virtually absent in others.

No simple or clear-cut mechanism for the formation of these complex structures could be deduced.