Antarctic Meteorite Research Center, National Institute of Polar Research
Abstract
A lot of troilite and/or magnetite rimmed FeNi metal grains have been found in 22 CO3 chondrites. The morphology of these grains is the most characteristic in opaque mineral assemblages in CO3s. These could be formed by reactions of FeNi metals with S-rich and/or O-rich gas. The number density of rimmed FeNi metals are correlated with subtype of CO3s. The grain size and the rim thickness of these grains are not significantly correlated with subtype. Magnetite is dominantly found in lower subtype (<3.2) and troilite is abundant but magnetite does not occur except Isna (3.6) and Ornans (3.3) in higher subtype (<3.2). In the subtype less than 3.2,troilite as inner rim and magnetite as outer rim could coexist for some rimmed FeNi metals (ALH-77307 and Y-81020). These textural variations were not formed by one series of thermal metamorphism but formed by (1) the differences of O/S conditions at the time of thermal metamorphism on the parent body, (2) oxidation from intermediate subtype to lower type and sulfidation from intermediate subtype to higher subtype, or (3) thermal metamorphism of rimmed FeNi metals especially in chondrules enclosed in mafic silicates at lower subtype formed in the solar nebula
