On the relationship between troilite and/or magnetite rimmed FeNi metals and subtype in CO3 chondrites

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 (