Stony irons mesosiderites

This chapter covers the major and minor achondrite groups, and three newly described unique achondrites. The discussion of other unique achondrites by Mittlefehidt et al. (1998) is stiU current. The silicates of the stony-iron mesosiderites show many similarities with the howardites, and the silicate inclusions in lAB irons are closely related to the stony winonaites. Therefore, these will be included here. Finally, some HE irons contain nonchondritic silicate inclusions, and they will also be considered. 

Stony Irons. The stony iron meteorites are composed of substantial iron and siUcate components. The paHasites contain cm-sized ohvine crystals embedded ia a soHd FeNi metal matrix and have properties consistent with formation at the core mantle boundary of differentiated asteroids. The mesosiderites are composed of metal and siUcates that were fractured and remixed, presumably ia the near-surface regions of their parent bodies. 

Stony irons are nonchondritic meteorites that contain roughly equal proportions of silicate minerals and metal. Two types of stony irons - pallasites and mesosiderites - are distinguished. Pallasites consist of approximately equal amounts of metal and olivine (one small group contains pyroxene as well). Mesosiderates also have approximately equal proportions of metal and silicate, but the silicate fraction is basalt. 

The stony-iron meteorites are intermediate between chondrites and irons. These very rare meteorites are equal mixtures of iron/nickel alloys and silicate minerals. Pallasites are striking examples of this type of meteorites, consisting of green olivine crystals in a matrix of metallic iron. Another type of stony-iron meteorite, called mesosiderites, contain pyroxene and plagioclase feldspars, minerals that are common on Earth. 

Mesosiderites are stony irons in which the rocky material is a polymict breccia of crustal rock from a differentiated body— basalt, gabbro, pyroxenite, dunite, and anorthosite. The silicates are very similar to the HED suite meteorites. Thus, mesosiderite silicates are discussed here, but the metallic phase is not. 

Most CRE ages and histories of pallasites rest shakily on noble gas analyses alone. We summarize the results in Figure 17. Pallasite exposure ages are longer than those of most stony meteorites and comparable to those of the other major stony iron group, the mesosiderites. 

Stony irons. Stony-iron meteorites are those which contain equal proportions of silicate minerals and metallic iron. Pallasites are made up of olivine and Fe-Ni metal and are thought to represent samples from the core-mantle boundary of their parent body. Mesosiderites are brecciated mixtures of silicates and Fe-Ni metal. 

The third large category of meteorites, the stony-iron meteorites, has traditionally been divided into two major groups, the pallasites and the mesosiderites, again based on their chemical composition. The pallasites consist of olivine crystals embedded in matrix of iron-nickel alloy, while the mesosiderites have a complex structure that includes pyroxene, plagioclase, olivine, and other minerals interspersed with an iron-nickel base. Some authorities now recognize a number of other classes of stony-iron meteorites that are different from the pallasites and mesosiderites in the kinds and amounts of minerals present. 

Figure 10. Exposure age distributions of iron meteorites and mesosiderite stony-irons. Iron meteorite data > 200 Myr were obtained by the method (Voshage 1978), ages < 200 Myr are based on...

Stony irons aubrites, ureihtes Pallasites, mesosiderites Pallasites derived from core-mantle... 

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