Low-temperature phase decomposition in iron-nickel metal of the Portales Valley meteorite

The Portales Valley meteorite provides an opportunity to investigate and co mpare the microstructure in Fe-Ni metal of the metallic particles in the ch ondritic portion and in the metal veins. The low-temperature phase decompos ition of Fe-Ni metal was investigated using scanning electron microscopy, t ransmission electron microscopy, and atomic force microscopy. The microstru cture is formed as the Portales Valley meteorite cooled from high temperatu res and includes the outer taenite rim, the cloudy zone, clear taenite, and martensite. Martensite in turn decomposes into a fine admixture of fee rod s in a bcc matrix. The width of the island phase of the cloudy zone in the metal particles of the chondritic portion and the metal veins can be used t o estimate a low-temperature cooling rate. The microstructural evidence ind icates that the chondritic portions and the metal veins in the Portales Val ley meteorite cooled together as a mixture with a cooling rate of roughly 6 .5 K/Ma.