The iron, magnesium, and calcium distribution in coexisting olivines and rhombic pyroxenes of chondrites

Iron, magnesium, and calcium in olivines and pyroxenes of 95 chondritic meteorites were analyzed with an electron microprobe. Of these, 86 were ‘Ordinary’ chondrites, each having constant iron-magnesium ratios in olivine and pyroxene. Between different meteorites these ratios vary within three narrow ranges which reflect the H- and L-group chondrites, as well as a third group, designated the LL group. The ratio of iron in olivine to that in coexisting pyroxene is nearly constant (∼1.1), particularly within the groups mentioned. A classification of stony meteorites based on the amount of iron in silicates and metal is proposed. It is shown that the iron content in the silicates is not related to the degree of metamorphism (recrystallization), and that therefore, the composition of the silicates was essentially established when the chondrules were formed. The ferrous iron also is not strictly related to the amount of metallic nickel-iron. Thus Prior's rule is not valid in its original sense, although it can be applied qualitatively when comparing the different groups of chondrites. The coexisting olivines and pyroxenes may not be in equilibrium in spite of their constant composition and the constant ratio between their iron contents. Furthermore, the metallic nickel-iron seems to have originated in the same melt as did the silicates.