Mineralogical Examination of the Allan Hills Achondrites and Their Bearing on the Parent Bodies

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Mineralogical examination of the Allan Hills achondrites collected by the U. S.-Japan joint team in the 1977 and 1978 seasons, except ALHA78113 and previously reported ALHA77005 and ALHA77302,revealed that they are polymict eucrites except two ureilites ALHA77257 and ALHA78019,and diogenite ALHA77256 and a primitive achondrite, ALHA77081. Among four polymict eucrites, ALHA78006 contains minor diogenitic components. The existence of such a polymict breccia can be explained in terms of excavation and mixing by impacts down to a very upper portion of the diogenite layer of a layered crust model of our howardite parent body. The MnO/FeO ratios of ALHA77081 lie on the extension of the equilibrated chondritic trend and the mineralogy is compatible with a primitive achondrite such as Winona. A coexisting pigeonite-orthopyroxene pair has been found in two Mg-rich Antarctic ureilites. On the basis of mineralogy of five Antarctic ureilites, an origin connected with a planetesimal produced by a fragmentation of the solar disk has been discussed.
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