Macrochondrules in Some Chondrites: 1. Structural-Mineralogical Characteristics

Abstract

The results of structural, mineralogical and chemical study of rare structural units of chondrites, macrochondrules and their fragments, found in five chondrites of different chemical groups and petrological types (Allende CV3, Krymka LL3.1, Saratov L4, "Velyka Balka" L4-5, Château-Renard L6), are given. Most of them are generally similar to ordinary chondrules and previously studied macrochondrules. They have a radial and porphyritic texture, consist of olivine and pyroxene, and are covered by a silicate rim, which is fine-grained in the macrochondrules of unequilibrated chondrites and coarse-grained in equilibrated ones. Only two macrochondrules among studied one’s are extraordinary and indicate specific conditions for their formation in the protoplanetary nebula. The first one, separated directly from the Allende chondrite, is characterized by the presence of a thin amorphous shell with a porous structure and with unambiguous sculpture features of instantaneous melting and solidification of its surface silicate layer. The second one, studied in a polished section of the Krymka meteorite, is characterized by a zonal structure and the presence of graphite grains and possibly bitumen inclusions. According to the SiO2/MgO ratio, its fine-grained silicate rim with rare graphite crystals and possibly bitumen inclusions corresponds to the fine-grained rims of ordinary chondrules, but is different from the carbonaceous material of meteorites.