A New Type of Antarctic Achondrites and their Relationship to S Asteroids and Chondrites

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Five meteorites recovered from Antarctica are unique achondrites with coarse-grained orthopyroxene and olivine crystals and variable amounts of Ni-Fe metal and FeS with additional augite or plagioclase. These mineral species can be found in chondrites, which are the most common types among the observed falls. Both Y74357 and MAC88177 contain considerable augite, but Y74357 is richer in olivine. Y791491 contains minor plagioclase. Discovery of such meteorites with coarse-grained texture and similar major mineral chemistry with extensively modified chondritic bulk chemistries suggest that they are related meteorites with variable amounts of augite and plagioclase and variable degree of reduction. The variability of mineral abundance can be explained by different degree of removal or segregation of partial melts, by a radiogenic internal heating and collisional one. Reflectance spectra of some members of this group combined with those of iron meteorites resemble those of S asteroids common in the main belt. The trend of their variation in mineral assemblage is in line with those of the S asteroids. This model also explains the absence of chondritic asteroids in the main belt, because S asteroids may be modified products of larger chondritic bodies.