Pallasites and mesosiderites

Olivine and metal are expected to be in contact at the boundary between a core and its overlying silicate mantle. That is the traditionally favored location for the formation of pallasites, although there are numerous problems with that explanation. 

Mesosiderites are a highly enigmatic group of differentiated meteorites. They are breccias composed of iron-nickel metal and silicate in roughly equal proportions. The metal represents molten material from the deep interior of an asteroid, whereas the silicate fraction consists of basalts and pyroxene cumulates similar to HED meteorites that formed near the surface. It is difficult to construct models that allow mixing of such diverse materials, but these disparate materials are generally thought to have been violently mixed by impact. 

The Apollo astronauts returned 382 kg of lunar sample to Earth, and this collection was supplemented by 326 g of soil samples collected by the Soviet Luna landers. The first lunar meteorite was found in 1982 in Antarctica. Since that time, over 120 lunar meteorites representing about 60 different fall events have been collected. The total mass of these meteorites is -48 kg. About one-third of these meteorites were recovered in Antarctica by American and Japanese teams, and most of the rest were recovered in the deserts of North Africa and Oman. The lunar meteorites have significantly expanded the areas of the Moon from which we have samples. 

The chemistry of lunar materials will be considered in much greater detail in Chapter 13. 

Martian meteorites ( SNC for shergottite, nakhlite, chassignite) comprise a diverse suite of igneous rocks (McSween, 2004). With only one exception, they are geologically young. Presently, 34 of these meteorites are recognized, most recovered from Antarctica and North African deserts. 

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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. 

Pallasite and mesosiderite Metal, troilite, schreibersite, graphite Coarse-grained, generally exsolved Fractional crystallization of core... 

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. 

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