Tissemouminites: A New Group of Primitive Achondrites

Primitive achondrites are a remarkable family of meteorites, marking the transition between unmolten chondrites and fully differentiated achondrites (Fig. 1). As such, these meteorites sampled partially melted planetesimals, formed only a few million years (My) after the formation of the first solids in the Solar System. Primitive achondrites have coarse-grained igneous achondritic textures, resulting from extensive thermal metamorphism, while retaining some chondritic features such as relict chondrules and chondritic bulk rock compositions (Krot et al. 2014). The peculiar nature of primitive achondrites constrains their parent body petrogenesis, which is yet to be fully understood. Primitive achondrites are subdivided into different groups on the basis of their petrologies and chemical and isotopic compositions, including the acapulcoites–lodranites, winonaites, brachinites, and ureilites (Krot et al. 2014). While brachinites and ureilites have very distinctive petrologic characteristics, the boundary between winonaites and acapulcoites is not so obvious. In fact, the similarities between winonaites and acapulcoites may have been hiding an entirely new class of primitive achondrites (Stephant et al. 2023).