Pure Fayalite in the Kaba Carbonaceous Chondrite

Details Pure Fayalite in the Kaba Carbonaceous Chondrite Pure Fayalite in the Kaba Carbonaceous Chondrite

Jul 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.236h&link_type=abstract

Meteoritics, vol. 27, no. 3, volume 27, page 236

Computer Science

1

Scientific paper

We found pure fayalite in both matrix and chondrules in the Kaba carbonaceous chondrite. This fayalite is closer to the end-member composition than any previously reported from meteorites. In the matrix it occurs in well-developed euhedral crystals up to 85 micrometers across that appear diamond shaped in thin sections. They occur as (a) isolated crystals surrounded by matrix, (b) groups of crystals having a core of troilite, and (c) crystals adjacent to clusters of troilite and magnetite. The fayalite in chondrules and chondrule-like objects is more abundant than in the matrix, and it occurs in several modes. It is present (a) in a large area (100 micrometers across) wrapping around one side of an enstatite chondrule and also within a thin (10 micrometers) rim; both chondrule and rim contain high concentrations of fayalite, magnetite, troilite and pentlandite; (b) in the outer part of a barred forsterite chondrule; this assemblage consists of Ni-Fe-sulfides, magnetite and minor fayalite; (c) in a chondrule-like object partially embedded within a Mg-rich chondrule devoid of fayalite; (d) in a rim (65 micrometers wide) associated with magnetite and troilite that surrounds a 200- micrometer core consisting of magnetite and dispersed troilite. The fayalite grains are almost pure, containing 99.5 mol% Fa where they are associated with troilite and magnetite. The isolated fayalite crystals in the matrix are zoned; their FeO contents decrease gradually from 70.6 wt% in the core to 67.2 wt% at the rim. MgO increases from 0.15 wt% in the core to 1.86 wt% at the rim and show a positive correlation with MnO, which ranges from 0.65 to 0.85 wt%. The associated sulfides are troilite and two Ni-bearing sulfides that contain 14.1 wt%, and 25.5 wt% of Ni, respectively. The coexistence of almost pure fayalite and pure Mg-rich silicates; the mineral assemblage of pure fayalite, magnetite, troilite and pentlandite in both matrix and in chondrules; and the chemical zoning in the isolated fayalite crystals in the matrix, particularly the positive correlation between MgO and MnO in these crystals, all indicate a complicated and intriguing formation history, making Kaba an extremely interesting meteorite to study.

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