Primitive achondrite
Primitive achondrites are meteorites that were heated enough to recrystallise and partly melt, but still keep near-chondritic bulk compositions.
Between chondrites, which never melted, and fully differentiated achondrites lies a grey zone. Primitive achondrites were heated to the point where iron-nickel metal and troilite began to melt together (at roughly 950–1,000 °C) and in many cases some basaltic melt formed and partly escaped. Their textures are recrystallised and achondritic, often with triple-junction grain boundaries, but their bulk chemistry and mineralogy remain close to a chondritic precursor, and some preserve relict chondrules.
The main groups usually included are the acapulcoites and lodranites (one parent body), the winonaites (linked to silicate inclusions in IAB irons), and the brachinites; ureilites are often added, though their origin is more complex. Taken together they record the earliest stages of melting and differentiation in planetesimals heated chiefly by the decay of aluminium-26, and how metal, sulfide and silicate melts separated.
The type example of its group, Acapulco, fell in Mexico on 11 August 1976. Recognition relies on texture under the microscope, mineral compositions, extent of melt depletion in rare earth elements and other trace elements, and, crucially, oxygen isotopes to assign a meteorite to a parent body.
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