THE FIRST FINDING OF CHONDRULE WITH AN UNORDINARY RIM IN THE KRYMKA METEORITE (LL3.1): MINERALOGICAL FEATURES AND GENETIC ASPECTS

Abstract

The results of structural, mineralogical and chemical studies of a chondrule with a fine-grained silicate-sulfide rim in the Krymka meteorite are given. A porphyritic texture, the presence of metal-troilite globules and the forsterite composition of olivine indicate іt`s high-temperature origin and belonging to a rare, but ordinary component of the Krymka meteorite. The chondrule is surrounded by an unusual wide (up to 200 μm) fine-grained rim, which is typical for chondrules of carbonaceous chondrites, but hasn`t been observed for ordinary chondrites. The rim contains high-temperature minerals, two generations of chromite grains, is essentially enriched in iron sulfide and is characterized by extreme xenomorphism of troilite grains, which attest their heating to the melting point, and by complete oxidation of troilite in inner and significant one in external zone of the rim. Supposition about oxidation of the iron sulphide in space rather than in terrestrial conditions has been made. According to SiO2/MgO ratio in a bulk chemical composition, the rim differs from silicate rims of chondrules, transparent and opaque matrix of the Krymka chondrite, but is close to its carbonaceous xenoliths. The data attests an accretion of dust by the chondrule in an area of fine-grained carbonaceous xenoliths formation and unlike that not of chondrule rims. The presence of a thick rim is an additional evidence of the origin and existence of the chondrule in a denser environment, enriched with a dust of a protoplanetary nebula different from other chondrules of the meteorite. The chondrule penetrated into agglomeration zone of a parent body of the Krymka meteorite with consolidated, compacted fine-grained rim, and relatively at low velocity, that contributed to its preservation in the meteorite.