Other
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.454c&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 454-455
Other
2
Chemical Composition, Chondrites, Inclusions, Meteoritic Composition, Mineralogy, Opacity, Abundance, Chemical Analysis, Grain Size, Meteoritic Microstructures, Olivine, Pyroxenes
Scientific paper
Axtell is a new CV3 chondrite from Texas. Preliminary studies of its Ca-Al rich Inclusions (CAIs) were reported. The mineralogy and chemical compositions of opaque minerals in four Axtell CAIs are distinct from their analogs in other inclusions from CV3 chondrites. Metallic NiFe (awaruite) is the dominant phase in opaque assemblages from Axtell inclusions, and occurs as well-rounded particles of sizes between about 1 and 20 microns. Their compositions (in atom %) are N63-67Fe31-33Co2-3 in AX-4, AX-7, and AX-9, and Ni74Fe23Co2 in AX-5. None of the awaruite grains contains measurable amounts (greater than 200 ppm by weight) of the platinum-group elements analyzed for Ru, W, Mo, Re, Os, Ir, and Pt. With very few exceptions, the awaruite particles studied are surrounded by or intergrown with V-free magnetite (detection limit = 400 ppm). In contrast with Allende, the dominant sulfide phase in Axtell inclusions is troilite (Ni less than or = 0.02-0.3%, by weight); pentlandite is scarce, occurring only sporadically as small grains associated with metallic FeNi (just a few particles of 5-15 microns were found in AX-5, AX-4, and AX-7, and none in AX-9). Also, an isolated 10-microns grain of millerite (NiS; Fe = 0.6 wt%) has been identified in AX-5. Unlike Allende inclusions, Axtell CAIs do not contain sulfide veins. Two remarkably large PGE-rich sulfide grains have been recognized: one spherical 12-microns particle in AX-4 (AX-4A, in melilite, close to the edge of the inclusion) and another irregularly shaped 25-microns grain in AX-9 (AX-9A, in melilite inside a spinel palisade). Study of the elemental distribution by wavelength-dispersive X-ray mapping and analysis and backscattered electron imaging suggests that these are single minerals and not multiphase assemblages. Microprobe analyses of chondrule olivines and low-Ca pyroxenes yield average Fa and Fs contents of 6.7 and 1.0 mol% respectively. These compositions suggest that Axtell is more similar to the CV3 chondrites of the reduced subgroup. The abundance of metal and scarcity of Ni-rich sulfides in Axtell inclusions are more similar to those in the inclusions of the reduced CV3 chondrites, although additional knowledge of CAI materials in oxidized CV3s other than Allende is of critical importance to fully assess this interpretation.
Casanova Ignacio
Simon Steven B.
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