Na-bearing Ca-Al-rich inclusions in the Yamato-791717 CO carbonaceous chondrite

Details Na-bearing Ca-Al-rich inclusions in the Yamato-791717 CO carbonaceous chondrite Na-bearing Ca-Al-rich inclusions in the Yamato-791717 CO carbonaceous chondrite

Jun 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27..136t&link_type=abstract

Meteoritics (ISSN 0026-1114), vol. 27, no. 2, June 1992, p. 136-143. Research supported by Mitsubishi Scientific Foundation.

Other

15

Aluminum, Calcium, Carbonaceous Chondrites, Inclusions, Meteoritic Composition, Nepheline, Ilmenite, Meteoritic Microstructures, Perovskites, Pyroxenes, Sodalite, Spinel

Scientific paper

Ca-Al-rich inclusions (CAIs) in the Yamato-791717 CO carbonaceous chondrite contain 5 to 80 vol pct of nepheline, along with minor sodalite, and thus are among the most nepheline-rich CAIs known. The primary phases in inclusions are mainly spinel, fassaite, aluminous diopside, perovskite, and hibonite. In contrast to many CO chondrites, melilite is rare. Spinel contains variable amounts of Fe (0 to 57 mol pct FeAl2O4) and is commonly zoned. Texture suggests that nepheline is a secondary alteration product formed by replacing mainly melilite, fassaite, and spinel; melilite is the most susceptible to alteration of the primary phases, so most of it was probably already consumed to form nepheline. The majority of inclusions are single concentric objects or aggregates of concentric objects. Lightly altered inclusions have cores of spinel surrounded by bands of nepheline (replacing fassaite), fassaite, and diopside. In moderately altered inclusions, spinel cores are replaced by nepheline. In heavily altered inclusions, the major part of internal areas (50 to 80 percent in volume) are replaced by nepheline. In some moderately and heavily altered inclusions, only diopside rims remain unaltered. Textural relationships indicate that the resistance of primary phases to alteration increases in the order melilite, fassaite, spinel, diopside. The alteration probably proceeded with reaction of the primary phases with the low-temperature (less than 1000 K) nebular gas rich in Na, Fe and Cl. The degree of alteration in Y791717 CAIs appears to be much higher than those in CALs in other reported meteorites.

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