Computer Science
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.204b&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 204
Computer Science
7
Scientific paper
In 1991 a single stone weighing 174 g was recovered from the Sahara desert, Algeria (27 degrees 36'N, 3 degrees 53'E). From this a 36.83-g fragment was obtained that has 2 sawn surfaces and adjacent surface of partially weathered fusion crust. The sawn surface shows "brownish" weathering products that have generally produced alteration of the matrix in which a few relict chondrules and inclusions are set. One 4 by 5 mm triangular area bordering the fusion crust is dark grey in colour with small inclusions set in it. This may be an area of local shock melt glass. Mineralogy and petrography: In a polished thin section, area 20 mm^2, we found few chondrules, mainly olivine clasts in a weathered olivine-rich matrix, with minor pyroxene. The largest relict chondrule was type "I" (McSween, 1977) with a diameter of 350 micrometers; a 200-micrometer-diameter type "II" (McSween, 1977) barred olivine chondrule showed devitrified glass in the interstices of the olivine. The sample is highly brecciated with fractures cutting the section and offsetting chondrule fragments, but in PTS is only very weakly shocked: olivine shows occasional undulatory extinction, i.e., stage "S2" in the classification of Stoffler et al. (1991). Mineral Chemistry: 95% of olivine is close to Fa(sub)39 but a few grains lie in the range Fa(sub)17 to Fa(sub)43 and one is zoned from a Fa(sub)17 core to a Fa(sub)43 rim. The majority of pyroxene is Ca-rich at a mean composition of Fs(sub)21 Wo(sub)45. Low-Ca pyroxene is also unequilibrated with a mean close to Fs(sub)31 and one has a composition of Fs(sub)10 Wo(sub)2. Plagioclase is a common interstitial phase in the matrix and is variable in composition, but high-Na oligoclase or albite. Minor opaque phases are dominated by Cr-rich (32-38 wt% Cr2O3) spinel, which also contains significant Ti (5.3 wt% TiO2). For charge balance substantial Fe^3+ is required indicating that Fe is partitioned between approximately 13-21 wt% Fe2O3 and 35 wt% FeO. The spinel therefore is a mixture between chromite, magnetite, and ulvospinel. Variation in the composition of this phase is principally in total Fe and Cr, which vary inversely. No Fe-Ni metal was found and troilite is extremely rare; the only analysis has trace Ni. Ni is undetectable in olivine and is depleted in the sample as a whole and this, together with higher mean Fa-values, would tend to rule out a relationship with CK-chondrites (Kallemeyn et al., 1990). Similarly, whilst the fayalite content in olivine might suggest a high-grade CO chondrite the lack of troilite, kamacite, and taenite and the abundance of Cr-rich spinel would tend to preclude this (McSween, 1976). Acfer 217 is a chondrite that is exceptionally poor in metal and sulfide and rich in oxidised Fe. It appears to be unique. Further work will include an oxygen isotope study to determine any relationship with known groups of meteorites. References: Kallemeyn G. W., Rubin A. E. and Wasson J. T. (1990) Geochim. Cosmochim. Acta 55, 881-892. McSween H. Y. (1976) Geochim. Cosmochim. Acta 41, 477-491. McSween H. Y. (1977) Geochim. Cosmochim. Acta 41, 1843-60. Stoffler D., Keil K. and Scott E.R.D. (1991) Geochim. Cosmochim. Acta 55, 3845-3867.
Bischoff Addi
Bland Phil
Hutchison Robert
Pillinger Colin T.
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