A 780 g stone was purchased in Morocco in 2000, and it was classified at Northern Arizona University as a howardite. The breccia comprises clasts of mostly diogenite composition (~82 vol%), with lesser amounts of basalts (~12 vol%) and shock-melt material (~4 vol%). The diogenite clasts consist primarily of orthopyroxene, along with pigeonite, anorthitic plagioclase, ilmenite, chromite, and pyrrhotite. This howardite has been shocked to stage S2, and has experienced only minor weathering to grade W1.
The basaltic achondrite group is a complicated one to classify due to the diversity in the structural and mineralogical relationships among its members. This group is composed of brecciated and unbrecciated, monomict and polymict eucrites, diogenites, and howardites, and has recently undergone a redefinition.
The monomict subgroup containing eucrites, cumulate eucrites, and diogenites is further subdivided into brecciated and unbrecciated members. The polymict subgroup samples a compositional and textural continuum of regolith and surface breccias consisting of eucrites, cumulate eucrites, diogenites, and howardites. Those meteorites containing more than 90% of a single component are given the prefix "polymict" attached to their present description (e.g. polymict eucrites contain less than 10% non-eucritic material; polymict diogenites contain more than 90% orthopyroxenite or hypersthene). Those meteorites that contain less than 90% of any single component are defined as howardites. While this 10% level is still an arbitrary dividing line based simply on mineral proportions, it represents an amount of orthopyroxene that can easily be detected by X-ray diffraction techniques. An additional tool to distinguish between polymict eucrites and howardites involves pyroxenes in the basaltic clasts; within howardites they are mostly unzoned whereas in polymict eucrites they are usually zoned.
A magnified view of this howardite reveals an abundance of yellow-green orthopyroxene crystals interspersed throughout the breccia. The NWA 1182 specimen pictured above is a 2.4 g partial slice.
