ACHONDRITE

any stony meteorite containing no chondrules (rounded granules of cosmic origin). Achondrites, constituting about 4 percent of the known meteorites, are similar in appearance to terrestrial igneous rocks low in silica content, such as basalts, peridotites, and pyroxenites. They may have crystallized from a melt similar to the terrestrial magma that formed basic and ultrabasic igneous rocks; it is possible that they were derived from the melting and fractional crystallization of parent bodies having the overall composition of chondrites. Achondrites are classified into the following groups according to their mineralogy: angrites, aubrites, chassignites, diogenites, eucrites, howardites, nakhlites, shergottites, and ureilites. See also chondrite.