Acapulcoites
General: The meteorites of this group are named for Acapulco, a primitive achondrite that fell in Mexico in 1976. Since its composition is near to chondritic, Acapulco was initially classified as an anomalous chondrite although it doesn't contain any chondrules. However, when similar stones were subsequently found, the acapulcoite group was formed to describe a distinct class of primitive achondrites, marking the transition between primordial chondritic matter and more differentiated rocks.
Description: At the first glance, the acapulcoites resemble metal-rich, highly equilibrated or recrystallized chondrites, with the difference that they are more coarse-grained, and that they don't contain any, or only a few relict chondrules. Micrometer to centimeter sized veins of nickel-iron metal are common. The fusion crust is usually dark brown or dull black, and gives no hint as to the true nature of the meteorite.
Mineralogy: Acapulcoites are primarily composed of fine-grained olivine, orthopyroxene, minor plagioclase, nickel-iron metal, and the iron sulfide, troilite. Their mineral compositions are intermediate between those of E and H chondrites, but they exhibit a characteristic oxygen isotopic pattern, distinct from all other known chondrite groups. Some members have been shown to contain relict chondrules, such as Monument Draw, and our new find, Oman 02067.
Formation history: The presence of distinct chondrules within these members attests to the fact that the acapulcoites represent an extremely primitive group, a true transition between chondrites and achondrites. Today, it is believed that the acapulcoites evolved from the partial differentiation of a small asteroid, and that melting and recrystallization of the acapulcoites occurred very early in the history of our solar system.
Origin: Asteroidal. It is thought that the acapulcoites originated on a small S-type asteroid, and that they share the same parent body with the closely related lodranites, another class of primitive achondrites. No spectral match for any specific asteroid has been identified, so far.