achondriteihin

Achondrites are a class of stony meteorites that lack chondrules, the small rounded grains found in primitive meteorites. They are igneous or metamorphic rocks formed by melting and differentiation within their parent bodies, and they typically preserve evidence of early planetary evolution. Because they do not contain chondrules, achondrites provide important clues about the processes of crust formation, melting, and differentiation in the solar system.

Origin and major groups: Most achondrites originated from differentiated asteroid bodies that experienced internal melting and recrystallization. The best-characterized group is the HED suite—Howardite, Eucrite, and Diogenite—linked to asteroid 4 Vesta. Eucrites are mostly basaltic rocks, diogenites are orthopyroxenites, and howardites are brecciated mixtures of eucritic and diogenitic material. Other important groups include angrites, which are ancient basaltic achondrites from a distinct parent body; ureilites, ultramafic, carbon-rich rocks with high olivine and pyroxene contents; and aubrites, enstatite-rich achondrites formed under low oxygen fugacity.

Additional sources: In addition to asteroid-derived achondrites, there are Martian and lunar achondrites among meteorite finds, representing igneous rocks formed on those bodies and subsequently delivered to Earth. These samples expand the record of planetary differentiation beyond the asteroid belt.

Mineralogy and dating: Achondrites typically consist of silicates such as plagioclase, pyroxene, and olivine, with little or no metallic chondrules. Radiometric dating shows that many achondrites formed within the first few tens of millions of years of the solar system, yielding ages around 4.5 to 4.56 billion years for some of the oldest fragments.

Significance: Studying achondrites helps scientists understand planetary differentiation, crust formation, and early solar-system history, complementing information from chondritic meteorites.