The olivine-metal textures of pallasites help elucidate the formation of these meteorites. The hexahedrites (low P members of chemical group IIAB) formed by the massive transformation through the reaction Оі в†’ О±m в†’ О± at relatively high temperature in the two-phase О± + Оі region of the Fe-Ni-P phase diagram near the О±/(О± + Оі) phase boundary. 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 Оі в†’ О±2 в†’ О± + Оі and Оі в†’ О±2 + Оі в†’ О± + Оі are responsible for the formation of plessite in irons and the metal phases of mesosiderites, chondrites, and pallasites. Depending on the Ni and P content of the meteorite, the kamacite nucleation temperature can be determined from either the (Оі + Ph)/(О± + Оі + Ph) boundary, the (О± + Оі)/(О± + Оі + Ph) boundary, or the Ms temperature.

We propose that the WidmanstГ¤tten structure in these meteorites forms by the reaction Оі в†’ О±2 + Оі в†’ О± + Оі, in which О±2 decomposes to the equilibrium О± and Оі phases during the cooling process.To determine the cooling rate of an individual iron meteorite, the appropriate formation mechanism for the WidmanstГ¤tten pattern must first be established. The formation of the WidmanstГ¤tten 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 О±2.

The reaction Оі в†’ О± + Оі, which is normally assumed to control the formation of the WidmanstГ¤tten pattern, is not applicable to the metal in meteorites. The WidmanstГ¤tten pattern in low P iron meteorites forms either at a temperature below the (О± + Оі)/(О± + Оі + Ph) boundary or by the decomposition of martensite below the martensite start temperature. These mechanisms can also be used to explain how the metallic microstructures developed in chondrites and stony-iron meteorites.The WidmanstГ¤tten pattern in high P iron meteorites forms when meteorites enter the three-phase field О± + Оі + Ph via cooling from the Оі + Ph field. This evidence includes the texture of Vermillion, siderophile trace element data, and the presence of cohenite in Vermillion.ДЄbstract- We have evaluated various mechanisms proposed for the formation of the WidmanstГ¤tten pattern in iron meteorites and propose a new mechanism for low P meteoritic metal. Although Vermillion and Y-8451 have similar mineralogy, pyroxene compositions, REE patterns, and O-isotopic compositions, there is sufficient evidence to resist formally grouping these two meteorites. The O-isotopic compositions of Vermillion and Y-8451 are similar but differ from Main Group or Eagle Station pallasites, as well as other achondrite and primitive achondrite groups. Rare earth element (REE) patterns in merrillite are similar to those seen in other pallasites, indicating formation by subsolidus reaction between metal and silicate, with the merrillite inheriting its pattern from the surrounding silicates. These compositions also differ from those of IAB and IIIAB iron meteorites. Siderophile trace element data show that metal in the two meteorites have significantly differing compositions that are, for many elements, outside the range of the Main Group and Eagle Station pallasites. Olivine compositions are Fa10вЂ“12, similar to those of Main Group pallasites. Symplectic overgrowths, sometimes found in Main Group and Eagle Station pallasites, are not seen in the pyroxene pallasites. Both meteorites contain low-Ca and high-Ca pyroxenes (<2% by volume) and have been dubbed вЂњpyroxene pallasites.вЂќ Pyroxene occurs as large individual grains, as inclusions in olivine and in other pyroxene, and as grains along the edges of olivine. Abstract- Two pallasites, Vermillion and Yamato (Y)-8451, have been studied to obtain petrologic, trace element, and O-isotopic data.

Seymchan meteorite