The formation of the Widmanstätten structure in meteorites

Abstract

We have evaluated various mechanisms proposed for the formation of the Widmanstätten pattern in iron meteorites and propose a new mechanism for low P meteoritic metal. These mechanisms can also be used to explain how the metallic microstructures developed in chondrites and stony-iron meteorites. The Widmanstätten pattern in high P iron meteorites forms when meteorites enter the three-phase field alpha + gamma + Ph via cooling from the gamma + Ph field. The Widmanstätten pattern in low P iron meteorites forms either at a temperature below the (alpha + gamma)/(alpha + gamma + Ph) boundary or by the decomposition of martensite below the martensite start temperature. The reaction gamma --> alpha + gamma, which is normally assumed to control the formation of the Widmanstätten pattern, is not applicable to the metal in meteorites. The formation of the Widmanstätten pattern in the vast majority of low P iron meteorites (which belong to chemical groups IAB-IIICD, IIIAB, and IVA) is controlled by mechanisms involving the formation of martensite alpha2. We propose that the Widmanstätten structure in these meteorites forms by the reaction gamma --> alpha2 + gamma --> alpha + gamma, in which alpha2 decomposes to the equilibrium alpha and gamma phases during the cooling process. To determine the cooling rate of an individual iron meteorite, the appropriate formation mechanism for the Widmanstätten pattern must first be established. Depending on the Ni and P content of the meteorite, the kamacite nucleation temperature can be determined from either the (gamma + Ph)(alpha + gamma + Ph) boundary, the (alpha + gamma)/(alpha + gamma + Ph) boundary, or the Ms temperature. With the introduction of these three mechanisms and the specific phase boundaries and the temperatures where transformations occur, it is no longer necessary to invoke arbitrary amounts of under-cooling in the calculation of the cooling rate. We conclude that martensite decomposition via the reactions gamma --> alpha 2 --> alpha + gamma and gamma --> alpha2 + gamma --> alpha + gamma are responsible for the formation of plessite in irons and the metal phases of mesosiderites, chondrites, and pallasites. The hexahedrites (low P members of chemical group IIAB) formed by the massive transformation through the reaction gamma --> alpha-m --> alpha at relatively high temperature in the two-phase alpha + gamma region of the Fe-Ni-P phase diagram near the alpha/(alpha + gamma) phase boundary.