Compositional heterogeneity of fine-grained rims in the Semarkona (LL3) chondrite

The fine-grained opaque matrix and chondrule rims in the Semarkona (LL3) chondrite have been investigated in detail with the scanning electron microscope. The chemical analyses were made by the SEM-EDS technique. Although the rims have similarities in texture and composition to the matrix, they show wider variabilities in chemical composition in comparison with the matrix in Semarkona. Based on both mode of occurrence and Al/Si ratio, the fine-grained opaque rims in Semarkona are classified into three types : type I, type IIa and type IIb. Type-I rims commonly occur as chondrule-rimming materials, having high Al/Si ratios. Type-I rims are composed mostly of smectite, showing a wider variation of the FeO and Al_2O_3 contents than that of the matrix. Type-IIa rims occur as magnetite-bearing chondrule rims, characterized by their low Al/Si ratios. They are composed of mixtures of smectite, fayalitic material and its alteration products. Metal-sulfide-magnetite aggregates have opaque rims (type IIb) with the lowest Al/Si ratios. The SiO_2,Al_2O_3 and FeO contents of type-IIb rims suggest that they consist mainly of fayalitic material and its alteration products. In compositon, it is suggested that \u27fayalitic\u27 material and its alteration products in types-IIa and IIb rims correspond to one of the non-refractory, SiO_2-, FeO-rich components of precursor materials of the chondrules in Semarkona. The \u27fayalitic\u27 material might have formed through intensive fractional condensation of forsterite in the cooling nebula gas