Mathematics – Logic
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29q.456c&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 456
Mathematics
Logic
4
Chondrites, Meteoritic Composition, Meteoritic Microstructures, Phase Transformations, Shock Loads, Silicates, Chemical Analysis, Crystal Structure, Grain Size, Mineralogy, Olivine, Pyroxenes
Scientific paper
The Sixiangkou chondrite (L6) is a rare example of heavily shocked chondrites. The meteorite is intersected by black silicate melt veins containing isotropic phases formed by shock transformation of silicates. Grains of olivine and pyroxene compositions in the veins are either isotropic mineral glass or ringwoodite or majorite respectively. Plagioclase has been transformed to isotropic glass. The phase tentatively identified as ringwoodite occures as isotropic and colorless large grains up to 300 microns in length usually with rounded outlines. Several sets of regular fractures dissect the grains. These structural features are considered to be related to the deformation process of olivine induced by the impact event. Numerous microprobe analyses indicate that the phase has the same composition as the birefringent olivine in the neighboring unmolten material. Three types compositional of the phase suspected to be majorite were encountered. Type 1 is colorless to yellowish brown and consists of isotropic polycrystalline aggregates ranging in size from approximately 20 to 150 microns. This type has the same composition as the low-Ca pyroxene of the chondrite mass. Type 2 has comparable optical properties to type 1, but the grains are smaller in size, ranging from approximately 5 to 20 microns. The main compositional difference between type 1 and type 2 is that the second one has higher Na2O contents. Type 3 consists of isolated yellowish brown microcrystals ranging in size from 1-3 microns. This phase occurs as subhedral or euhedral crystals in the matrix of veins, and is usually surrounded by fine-grained troilite and FeNi metal or plagioclase glass. Microprobe analyses indicate that this type is relatively rich in Al, Ca, Na, and Cr. Based on the mineralogical features of the veins, we conclude that the shock-induced phase transformation of minerals may have been produced in different stages of a shock event.
Chen Michelle
El Goresy Ahmed
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