Schreibersite Growth and Its Influence on the Metallography of Coarse-Structured Iron Meteorites

Abstract

The role that schreibersite growth played in the structural development process in coarse-structured iron meteorites has been examined. The availability of many large meteorite surfaces and an extensive collection of metallographic sections made it possible to undertake a comprehensive survey of schreibersite petrography. This study was the basis for the selection of samples for detailed electron microprobe analysis. Samples containing representative structures from eight chemical Groups I and IIAB meteorites were selected. Electron microprobe traverses were made across structures representative of the observed range of schreibersite associations. Particular emphasis was placed on schreibersite-kamacite interface compositions. An analysis of these data has led to a comprehensive description of the structural development process. Massive schreibersite, one of the four major types of schreibersite encountered, may be accounted for by equilibrium considerations. Subsolidus nucleation and growth with slow cooling from temperatures at least as high as 850? C, and probably much higher, explain the phase relationships that one sees in meteorite specimens. The retention of taenite in the octahedrites establishes that bulk equilibrium did not extend as low as 550? C. Schreibersite undoubtedly continued in equilibrium with its enclosing kamacite to lower temperatures. A second type of schreibersite to form is homogeneously nucleated rhabdite. It nucleated in kamacite in the 600? C temperature range, either as a consequence of low initial P level or after local P supersaturation developed following massive schreibersite growth. A third type of schreibersite is grain boundary and taenite border schreibersite. It formed at kamacite-taenite interfaces, absorbing residual taenite. Nucleation took place successively along grain boundaries over a range of temperatures starting as high as 500? C or perhaps slightly higher. Grain boundary diffusion probably became an increasingly important factor in the growth of these schreibersites with decreasing temperature. The fourth type of schreibersite is microrhabdite. These schreibersites nucleated homogeneously in supersatuated kamacite at temperatures in the 400? C range or below. P diffusion controlled the growth rate of schreibersite. The Ni flux to a growing interface had to produce a growth rate equal to that established by the P flux. This was accomplished by tie line shifts that permitted a broad range of Ni growth rates, and these shifts account for the observed range of Ni concentrations in schreibersite. Equilibrium conditions pertained at growth interfaces to temperatures far below those available experimentally. Kinetic factors, however, restricted mass transfer to increasingly small volumes of material with decreasing temperature.