Structural development in the Santa Catharina meteorite

A metallographic study was undertaken to seek support for the idea that Santa Catharina is really massive cloudy taenite. Available metal-rich nuggets of Santa Catharina appear to have been single crystals of taenite in the few centimeter size range, separated from each other by grain boundaries occupied by troilite and schreibersite. Metallographic and electron microprobe data allow one to postulate the following cooling and structural development history: (1) single crystal taenite formed at high temperature; (2) phosphate formed within the taenite and grain boundary schreibersite formed at interfaces with troilite or with other taenite crystals; (3) schreibersite began to precipitate within taenite at about 650 C; (4) at about 450 C the meteorite entered the three phase field at which point kamacite precipitated and started growing; and (5) kamacite/schreibersite interface measurements indicate that cooling continued down to about 350 C, with large Ni diffusion gradients developing within schreibersites.