Metallography of Chaunskij, Russia, Mesosiderite

Details Metallography of Chaunskij, Russia, Mesosiderite Metallography of Chaunskij, Russia, Mesosiderite

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.337c&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 337

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Chaunskij, Mesosiderite, Metallographic Structure

Scientific paper

Chaunskij is an unusually metal-rich meteorite with metallographic structures indicating a typical mesosiderite cooling history. Its metal is unusually Ni-rich, and it has been subjected to higher shock levels than other mesosiderites. Bulk metal composition in wt% is Ni 13.10, Co 0.54, and P 0.10. Troilite is abundant compared to other mesosiderites and schreibersite is rare. Silicates probably comprise less than 20 wt% of the meteorite. Equant to slightly elongated kamacite areas in the mm-size range are the dominant mineral on polished surfaces. Kamacite grains have been strongly shocked and display varying intensities of epsilon-structure, apparently depending on the orientation of the grain. At exterior edges of the specimen, a pronounced alpha(sub)2 heat-altered zone a mm or more in width is superimposed on the epsilon-structure. Recrystallized areas of kamacite 20 micrometers or smaller on an edge, some with a distinct alpha-2 structure, occur throughout the surface. They are particularly abundant at boundaries with taenite and in areas of kamacite where the epsilon-structure is not strongly developed. Cloudy taenite regions bordered by 10-20-micrometer wide rims of 47-53% Ni taenite are the second major feature of the structure. In areas of structure showing the least indications of shock effects, the taenite frequently displays the typical optical anisotropy of tetrataenite, indicating that the cloudy taenite was bordered by tetrataenite that has since been shock disordered. Traverses across two large cloudy taenite areas show a gradual decrease in Ni content to low points of 35 and 29% Ni. The cloudy taenite fields have a martensite-like texture that may well be a shock-induced feature. In a small number of cloudy taenite areas, Ni values appear to be low enough that some true martensite is present. Unevenly distributed, shock-twinned troilite is an important feature of polished surfaces, making up about 5% of the area. Globular masses are in the mm-size range with some few having surface areas as large as 25 mm^2. Schreibersite is distributed in minor amounts along kamacite grain boundaries and as very small crystals at taenite interfaces.

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