The polymict eucrite Macibini is a fragmental breccia predominantly composed of eucritic materials with minor proportions (about 2 vol. %) of diogenitic material. Hence, it is intermediate between the Y74159-type polymict eucrites, which contain negligible amounts of magnesian orthopyroxene, and the howardites. The present study provides mineralogical and bulk compositional data for the brecciated matrix of the meteorite and for six clasts. These clasts include fragments of volcanic/igneous rocks and earlier formed breccias, including an impact melt breccia composed of rock and mineral fragments in a devitrified glassy matrix. Small pieces of devitrified brown glass are also present in the matrix of the meteorite and are interpreted as impact generated. The most unusual clast found in Macibini is clast A/B, which is ophitic/subophitic in texture with blocky feldspars, strongly zoned pyroxenes, and large laths of a silica mineral. The edges of the zoned pyroxenes in this clast are made up of a host of iron-rich augite containing vermicules (blebs) and lamellae composed of a mixture of olivine and silica. Similar features have been found as fragments in lunar breccias and have been attributed by some workers to the breakdown of metastable pyroxferroite to a mixture of augite, olivine, and silica. Textures and compositions of the features in clast A/B indicate that they are the result of decomposition in a series of steps of metastable, Fe-rich subcalcic augite. The presence of these features at the edges of zoned pyroxene grains in an unequilibrated eucrite clast indicates that they can result from closed-system crystallization of a relatively mafic melt.