The formation of clear taenite in ordinary chondrites

Details The formation of clear taenite in ordinary chondrites The formation of clear taenite in ordinary chondrites

Feb 1971

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1971gecoa..35..175t&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 35, Issue 2, pp.175-176

Computer Science

5

Scientific paper

The metallic phases in six bronzite and six hypersthene chondrites were studied metallographically and by electron microprobe. All of the chondrites studied contain zoned taenite. In bronzite chondrites, only about 5 per cent of the zoned taenite abuts on kamacite (the rest being apparently isolated from it) whereas in hypersthene chondrites an average of over 20 per cent abuts on kamacite. The compositions of the centers of zoned taenite can be used to obtain cooling rates by Wood's method. Including Wood's results, 14 out of 18 ordinary chondrites have cooling rates between 1 and 10°C/m.y. In addition to zoned taenite, all of the bronzite chondrites contain clear taenite which nearly always borders on kamacite and contains 50-57 wt. % Ni. There is a slight Ni gradient in the clear taenite such that the Ni content increases on the average by 2 % Ni from silicate to kamacite borders. Clear taenite is also found in three of six hypersthene chondrites, but it is significantly less abundant in them than in the bronzites. The composition suggests that clear taenite formed in the temperature range 390 - 340°C. Apparently it nucleated on pre-existing kamacite when the diffusion of Ni from kamacite to zoned taenite had become too sluggish to maintain equilibrium. This explanation may account for the fact that clear taenite is much more abundant in bronzites than in the hypersthenes where zoned taenite frequently borders on kamacite.

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