Studies have shown that trace element contents as well as platinum group elements within the metal fraction of group IAB meteorites are highly variable on a sub-mm scale. This inhomogeneity requires a low-temperature origin since a high-temperature origin from an igneous melt, followed by such slow cooling rates, would have homogenized the metal. Additional chemical and isotopic constraints lead to the conclusion that the metal was probably formed by a chemical vapor deposition process. The graphite and other silicates were then combined with the metal fraction while in the solid state. In support of a low-temperature history for the graphitemetal inclusions is the presence of various noble gas components (Matsuda et al., 2005). Furthermore, the presence of excess 129Xe in the graphitemetal inclusions is consistent with their formation and inclusion within the metal host while the nebula was still primitive. Green mineral crystals, either kosmochlor (NaCrSi2O6) or krinovite (NaMg2CrSi3O10), were identified in a graphite nodule by Dr. Laurence Garvie.
Kosmochlor or krinovite crystals in a Canyon Diablo graphite nodule; FoV 2.5 mm
Photo shown courtesy of Dr. Laurence Garvie, Center for Meteorite Studies, ASU
These graphite nodules survived the incredible 40-megaton blast that occurred around 50,000 years ago in Arizona, destroying 99.999% of the half-million ton mass and creating a spectacular crater. Extreme heat and pressure forced metal in the form of kamacite into the graphite nodules creating ribbons of metal in a jet-black carbon matrix. The above specimen is a 47.3 g end section. Below is a photo of the impact-melted backside of this specimen.