Y-790727, 52-2: Howardite

         This specimen was originally listed as a polymict eucrite in the first catalog of the Yamato-79 collection (Kojima and Yanai, 1981), but greenish yellow pyroxenes exposed on the surface of the specimen were confirmed to be orthopyroxene of the howardite-type by single crystal X-ray diffraction. The bulk chemical compositions plot in the middle of the diogenite-eucrite trend in the Al₂O₃ vs. CaO diagram, and is close to that of cumulate eucrite, Binda.

         This is an angular stone covered by black shiny fusion crust. Irregular deep pits are distributed unevenly, where a slightly stained gray matrix of fine mineral fragments can be seen. At the bottom of a large hole, a relatively coarse lithic clast is observed. Similar vugs have been described in Allan Hills collections.

         The thin section is rich in mineral fragments and shows complex breccia of angular fragments, up to 1.7 mm long, of orthopyroxene, inverted pigeonite, pigeonite, olivine, and plagioclase, with a few lithic clasts, set in a matrix of comminuted pyroxene, olivine and plagioclase. This texture and mineralogy are typical of howardites. Accessory chromite and ilmenite and trace amounts of troilite and nickel-iron are present. The lithic clasts are holocrystalline orthopyroxenite and pyroxene-plagioclase aggregates, and range in texture from dunite, coarse-grained orthopyroxenitic and gabbroic to fine-grained basaltic types. Dark aphanitic clasts are also present.

         Microprobe analyses show a wide range in pyroxene composition. Pyroxenes similar in composition to those in diogenite and eucrites are common, but some pyroxene compositions extend beyond the most Mg-rich diogenite to nearly Ca₁Mg₈₁Fe₁₇, and there are inverted pigeonites with Fe/(Mg+Fe) between 0.3 to 0.6. The olivine compositions range from Fa₁₁ to Fa₂₉. Plagioclase ranges from An₉₄ to An₈₂ and averages An₈₈. The presence of orthopyroxene has been confirmed by the single crystal X-ray diffraction. The largest clast in the thin section, 1.7x1.4 mm in size, shows a fine-grained granoblastic texture of orthopyroxene and includes minor fine chromite grains. The texture is similar to some portions of the Y-74013-type diogenites with granoblastic texture. Another recrystallized orthopyroxenite clast 0.8x0.7 mm in size, contains very fine rectangular to lath shaped crystals, but there remains in one area, an unrecrystallized coarse orthopyroxene crystal. No olivine was present. The fine texture suggests a part of orthopyroxene was molten and cooled rapidly to produce the fine-grained texture. More Mg-rich compositions of the melted portion are in agreement with this interpretation.

         A basaltic clast 0.53x0.36 mm in size consists of slightly radiating subparallel laths of alternating pigeonite and plagioclase. The pyroxenes with the eucritic composition show augite exsolution with (001) in common with the host.