Y-791438, 51-1: Eucrite (magnesium non-cumulate eucrite)

          Y-791438 is a crystalline, magnesian non-cumulate eucrite. Y-791438 is composed mainly of plagioclase (36 vol.%) and pyroxene. Minor minerals include ilmenite, chromite, troilite, Ca phosphates and zircon. The texture of Y-791438 is slightly disturbed by shock events, but the original texture seems to have been ophitic with the sizes of pyroxene ranging from 0.5 to 1 mm in length and those of plagioclase from 0.2 to 0.8 mm. Pigeonite crystals show fine exsolution lamellae (width 1 to 2 microns) of augite on (001). A large grain of pigeonite may be partly inverted to orthopyroxene in different orientations, but this texture is different from that of cumulate eucrites in that the width of exsolution lamellae of cumulate eucrite is more than 10 microns and orthopyroxene crystals grow in much larger scale. There is another portion in Y-791438, with very different appearance indicating a secondary shock event. About a half of the pyroxene grain-boundaries are covered with fuzzy glassy materials with chemical compositions intermediate between augite and plagioclase. Globules of Fe-S ranging from 0.5 mm to 1 mm microns in diameter often observed in such areas. The bulk composition of the pigeonite is Ca₈Mg₅₀Fe₄₂, which is similar to those of cumulate eucrites. Most of the plagioclase crystals shows slight normal zoning. The An contents of plagioclase range from An₉₅ to An₉₀. The range of An is smaller than that in ordinary eucrites, but larger than those of cumulate eucrites.