Computer Science
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.600w&link_type=abstract
Meteoritics, vol. 30, no. 5, page 600
Computer Science
Clasts, Ci Chondrites, Differentiation, Dolomite, Orgueil, Magnetite, Meteorites, Parent Body
Scientific paper
In a thin section of CI Orgueil a 4x5 mm clast was found with an unusual composition. It consists mainly of magnetite (63 wt%) and dolomite (12 wt%) in the usual fine matrix. The magnetite occurs in rounded grains of 5 to 20 micron, occasionally also as framboidal spherules composed of many tiny grains. The dolomite grains have about the same size and rounded shape as the magnetites. The magnetite is chemically very pure, with no element except iron detectable by microprobe (as usual in Orgueil). Dolomite contains, besides Ca and Mg, variable amounts of Mn and Fe. In a Mn vs. Fe diagram they plot in the same area as reported for Orgueil [1], but they show an Fe/Mn anticorrelation, which has not been found before. Mn varies over a wider range than Fe (Fig. 1), so there is not a simple replacement of Mn by Fe, or vice versa. Many dolomite grains show fan-shaped overgrowth towards the matrix; these areas plot at the low-Mn/high-Fe end of the Fe/Mn anticorrelation (Fig. 1). Dolomites sometimes contain tiny spherules of magnetite, often arranged in strings close to their outer margins. The bulk composition of this clast shows more than twice as much Fe and three times as much Ca as Orgueil, and is correspondingly poorer in Mg and Si (Table 1). The matrix composition in the clast is generally similar to that in Orgueil proper, but it is significantly higher in Fe, S, and Ni. The similar compositions of magnetite and dolomite in the clast and in Orgueil proper show that they formed under similar conditions, that is on the parent body [1, 2]. The high enrichment of these minerals must have formed by differentiation on the parent body. This differentiation was no longer isochemical, as so far assumed for all processes on the Orgueil parent body. References: [1] Fredriksson K. and Kerridge J. F. (1988) Meteoritics, 23, 35-44. [2] Johnson C. A. and Prinz M. (1993) GCA, 57, 2843-2852. Table 1: Bulk composition of clast and Orgueil proper Clast^1 Orgueil^2 MgO 11 22 Al2O3 0.5 2.3 SiO2 10 31 S 2.0 7.6 CaO 6.1 1.9 FeO* 69 33 NiO 1.3 1.8 100 100 1 Calculated from mineral compostion and mode. 2 Recalculated from chemical analysis by E. Jarosewich in [1]. * All Fe as FeO.
Fredriksson Kimmo
Thacker Robert
Wlotzka Frank
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