Primitive achondrites and ureilites

Between chondrites and fully melted achondrites lie rocks caught in the act of melting. They reveal how differentiation begins.

Acapulcoites and lodranites

Acapulcoites are named after Acapulco, which fell in Mexico in 1976, and lodranites after Lodran, which fell in what is now Pakistan in 1868. They are thought to come from the same parent body, because they share oxygen isotope compositions and grade into each other.

Acapulcoites are fine-grained, roughly chondritic in composition, and show evidence of a small degree of partial melting; rare relict chondrules survive in some. Lodranites are coarser grained and depleted in the earliest melts, iron-nickel-sulphide and plagioclase-rich basaltic components, indicating higher degrees of melting and melt extraction.

Together, they show a single body heated progressively from metamorphism into the onset of melting. This sequence provides rare natural evidence for how melts first separate from a chondritic source, the process that, taken further, leads to cores and basaltic crusts.

Winonaites and brachinites

Winonaites are named after Winona, found in Arizona in 1928. They are fine- to medium-grained, roughly chondritic rocks with signs of partial melting. Their oxygen isotopes and mineral compositions closely match the silicate inclusions in IAB iron meteorites, implying a shared parent body in which metal and silicate were incompletely separated, perhaps disturbed by impacts.

Brachinites, named after Brachina in South Australia, found in 1974, are olivine-rich rocks with relatively iron-rich olivine. They are interpreted as residues of partial melting or as cumulates. Several brachinite-like ungrouped achondrites have been described, and the relationships among them remain under study.

These groups demonstrate that partial differentiation was common, and that many parent bodies did not complete the transition to a fully layered structure.

Ureilites

Ureilites, named after Novo-Urei, which fell in Russia in 1886, are the second-largest group of achondrites. They are ultramafic rocks made mainly of olivine and pigeonite, with up to several per cent carbon as graphite and diamond, and some metal and sulphide.

They are interpreted as mantle residues from which basaltic melts were removed. Unusually, their olivine core compositions correlate with Δ17O, which varies along the CCAM line. This means the parent body never fully homogenised its oxygen isotopes, despite extensive melting.

The ureilite parent body appears to have been disrupted catastrophically while still hot, causing rapid cooling and reduction recorded in thin rims on olivine grains. Polymict ureilites are breccias of many fragments.

The diamonds are debated. Most are thought to form by shock, but large diamonds with inclusions in some Almahata Sitta samples have been interpreted as growing at high static pressure in a large parent body.

Almahata Sitta

The Almahata Sitta meteorites, from asteroid 2008 TC3, transformed ureilite research. Most fragments are ureilites, but a minority are enstatite chondrites, ordinary chondrites, carbonaceous chondrite material and other types.

This showed that a small asteroid, a few metres across, can be a rubble pile containing debris from many sources, mixed by impacts. Before Almahata Sitta, such heterogeneous asteroids were known mainly from breccias, but the scale of mixing in a single small body surprised many researchers.

Because the asteroid was observed spectroscopically before impact, Almahata Sitta also links a specific meteorite assemblage to a measured reflectance spectrum, an F-class object. The fall has become a reference case for interpreting the surfaces and interiors of small near-Earth asteroids. It also explains why such a mixed asteroid would be hard to recognise from a single meteorite.

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