Shyrinbekova S.N. Influence of terrestrial weathering on the structural and mineralogical features of meteorites. – Manuscript.
Thesis for a Candidate Degree in Geological Sciences. Speciality 04.00.20 – mineralogy, crystallography. – M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of the National Academy of Science of Ukraine. – Kyiv, 2019.
The results of the first systematic electron microscopic study of the terrestrial weathering products in different types of meteorites from the meteorite collection of the National Museum of Natural History of the National Academy of Sciences of Ukraine allow us to draw conclusions that are consistent with the literature data on the problem of terrestrial weathering of the meteoritic matter.
1. Structural, mineralogical and chemical changes of meteorites in the terrestrial environment are determined by internal and external weathering factors. The internal factors are related to the structure, chemical and mineral composition of the meteorites. The weathering of meteorites is facilitated by the structural heterogeneity – Widmanstätten patterns, found in the octahedrites, chondritic texture of stone meteorites, Fe-Ni matrix of pallasites. High content of Fe-Ni metal in meteorites and low concentration of Ni in Fe-Ni metal are determinant for the weathering process. The similarity of the nature of meteoritic Fe-Ni alloy weathering of different meteorite types was revealed. The selective corrosion of Fe-Ni metal is the main cause of weathering of meteorites of all types.
The external factors of meteorite terrestrial weathering are the following: whether the meteorites belong to falls or finds, their terrestrial age, climatic conditions in the area of finding, laboratory and museum storage conditions.
2. The weathering of iron meteorites is caused by: a) the structural features, namely, the coarse and coarsest structures of the Kaalijarv and Sikhote-Alin octahedrites, respectively, Widmanstätten pattern of Fe-Ni metal and plessite fields in their matter, thin plessite structure of Chinga ataxite; b) low Ni content in octahedrites; c) mineral inclusions, d) shock-metamorphic effects in meteorites. The meteoritic matter alteration was facilitated by the local environment conditions and the significant terrestrial ages of the Kaalijarv and Chinga meteorite finds. In the metal plates from Chinga meteorite, a spatial image of the selectively weathered extremely thin relic taenite structure of Ni-rich ataxites was received for the first time.
3. For the first time, ordinary chondrites from the meteorite collection of the NASU have been classified in accordance with a weathering scale for ordinary chondrites (by F. Wlotzka, 1993) predominantly as unaltered of the W0 weathering grade and minor altered of W1 grade, as well as moderately altered of W2 and heavy altered of W3 grade. The 21 chondrite matter preserved pre-terrestrial structural, mineralogical and chemical features that can be used to investigate the conditions of formation and evolution of the meteoritic matter in the space. The weathered matter of ordinary chondrites that belong to the W2–W3 stages can be used for interpretation of the pre-terrestrial processes only with certain restrictions.
The conclusion of other researchers that under terrestrial conditions the meteorites of low petrologic types are weathered faster than the meteorites of high petrologic types was confirmed (LL group).
4. Weathering of pallasites occurs through the Fe-Ni matrix corrosion and due to shock metamorphism of minerals. Occurrence of thin morphologic types of weathering products on the olivine grains are caused by kamacite corrosion.
5. The morphological diversity of the weathering products of meteorites is genetically associated with oxidized kamacite, to a lesser extent with taenite, troilite, and, to a small extent, with the inclusions of phosphides, copper and silicates. Iron oxide and oxyhydroxide phases dominate among weathering products, less spread are secondary Сu sulphides.
6. For the first time, the morphologic classification of meteorite terrestrial weathering products has been proposed based on the electron microscopic data. Three morphological types: complanate, isometric and elongated ones describe the spatial location and morphology of the secondary phases in meteorite samples.
7. Nano- and micrometer secondary crystals were formed in the microcavities of the weathered meteorite samples. They are characterised by increased adsorption capacity that contributes to further oxidation of the meteoritic matter under terrestrial conditions.
8. Our study has shown that to ensure the long-term storage of the meteorite samples in the museum collection, it is necessary to monitor the chlorine content in the weathering products, humidity and air pollution.
Key words: meteorite, octahedrite, ataxite, chondrite, pallasite, Fe-Ni metal, troilite, terrestrial weathering, limonite, akaganeite, goethite.
