PORPHYRITIC BITUMEN-BEARING XENOLITH WITHIN THE KRYMKA (LL3.1) CHONDRITE: 1. STRUCTURAL-MINERALOGICAL CHARACTERISTIC

UDC 523.681

https://doi.org/10.15407/mineraljournal.38.03.056

Girich А.L., Semenenko V.P.
M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of the NAS of Ukraine
34, Acad. Palladina Pr., Kyiv-142, Ukraine, 03680
E-mail: cosmin@i.ua
PORPHYRITIC BITUMEN-BEARING XENOLITH WITHIN THE KRYMKA (LL3.1) CHONDRITE: 1. STRUCTURAL-MINERALOGICAL CHARACTERISTIC
Language:
Ukrainian
Mineralogical journal 2016, 38 (3): 56-66

Abstract: The results of structural-mineralogical and chemical study of a new xenolith within the Krymka (LL3.1) chondrite are given. The xenolith is characterized by coarse-grained porphyritic structure, the presence of coarse inclusions of bitumen-bearing material that are located predominantly on its periphery, fine-grained polymineral associations of troilite, silicate material predominantly of olivine composition, phosphates and nickel iron, first identified in meteorites disthene (?) probably of metamorphic nature, heterogeneous composition of minerals, especially olivine, Ca-low pyroxene and chromite, complicated zoning of some Ca-low pyroxene grains, indicating to rhythmic changing of redox conditions during its crystallization, and high Al2O3 and MgO content in chromite, indicating to the high-temperature conditions of formation. The main minerals of xenolith are olivine (Fa6.83—97.0), Ca-low pyroxene (Fs2.64—22.6En75.3—96.7Wo0.50—6.21) and troilite. The secondary ones are Ca-rich pyroxene (Fs3.98—8.27En55.0—63.6Wo28.9—39.0) and nickel iron (taenite and kamacite). Accessory minerals are chromite, phosphates (merrillite and rare for chondrites panetite) and disthene (?). Mesostasis residues are preserved in an intergranular space, its chemical composition corresponds to the normative plagioclase (Ab69.1—82.3An16.1—30.8Or0.15—2.55). The xenolith is surrounded by a fine-grained rim composed of cryptocrystalline silicate material, fine grains of olivine (Fa4.86—65.6), enstatite (Fs2.16—2.91En96.8—97.4Wo0.03—0.49), Ca-rich pyroxene (Fs5.67—10.3En45.5—54.6Wo39.7—44.2), kamacite, taenite, troilite and carbon-bearing material. Microchondrules and clasts of fibrous olivine crystals are present within the rim. According to microprobe study a bulk chemical composition of the rim determined as an average of 117 analysis is the following (wt. %): 40.6 FeO; 32.2 SiO2; 11.9 MgO; 3.38 Al2O3; 1.28 Na2O; 0.98 Ni; 0.72 CaO; 0.36 MnO; 0.27 Cr2O3; 0.24 К2O; 0.14 P2O5; 0.19 S; total — 92.2. The SiO2/MgO ratio corresponds to 2.7, and FeO/(FeO + MgO) — 0.77. Porphyritic bitumen-bearing xenolith differs from the known Krymka xenoliths indicating to its most probable formation in result of unequilibrated crystallization of a silicate melt.

Keywords: meteorite, chondrite, xenolith, bitumen, porphyritic structure, minerals, fine-grained material.

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