Iron, IAB complex, Twin City duo
standby for santa catharina photo
Found 1875
26° 19' S., 48° 39' W.

Many large masses of this ungrouped, Ni-rich (35.3%), troilite-rich, ataxite were found on and within a small hill by Gonclaves da Rosa on the island of São Francisco, Santa Catarina, Brazil. The combined weight of the recovered masses was estimated to be at least 7,000 kg, but was probably much higher since a large portion was smelted to extract its unusually high content of nickel (some portions up to 50%). Although the IAB-complex irons are the products of impact-related melting, the anomalously high Ni content in Santa Catharina is more consistent with fractional crystallization within a deep melt pool.

Although much of this iron has been significantly oxidized by terrestrial weathering, some metallic portions are preserved. The presence in Santa Catharina of the phosphide barringerite, a mineral usually unstable in association with metal, attests to a high cooling rate; however, a low cooling rate is considered necessary for the formation of some unique FeNi-metal phases. Those FeNi-metal mineral phases present in Santa Catharina include an ordered Fe-deficient tetrataenite (44.6 wt%) associated with a disordered paramagnetic, Fe-rich antitaenite (48.9 wt%), and a disordered Fe-deficient, face-centered cubic phase (6.5 wt%); these phases were identified by synchrotron X-ray diffraction techniques (Scorzelli et al., 2007). Based on noble gas data, Santa Catharina has a CRE-age of 140 m.y.

Santa Catharina shares many taxonomic characteristics with Twin City, which prompted Wasson and Kallemeyn (2002) to classify them as a unique duo within the IAB iron complex. Twin City contains slightly less Ni (30%) and troilite than Santa Catharina, and instead of an ataxitic structure, it exhibits a rudimentary orientation of kamacite spindles into a Thomson (Widmanstätten) structure. Although to a much lower degree than Santa Catharina, Twin City has suffered significant deterioration from thousands of years of terrestrial weathering. The specimens of Santa Catharina pictured above are a 0.3 g unoxidized thin partial slice (left) and a 6.84 g oxidized fragment (right; specimens not shown to the same scale). The bottom photo is a grouping of rare unoxidized fragments shown courtesy of Dr. Jay Piatek. A close-up view of a large unoxidized specimen exhibited at the Muséum National d'Histoire de Paris can be seen at their website.

standby for santa catharina photo
Photo courtesy of Dr. J. Piatek