Lunar Meteorite: Yamato 791197

Yamato Mountains, Antarctica

Four views of Yamato 791197. This is one of several lunar meteorites that are regolith breccias with highly vesicular fusion crusts. Photos courtesy of the National Institute of Polar Research, Japan

Two views of lab sample of Y-791197. Millimeter ticks for scale. Photo credit: Randy Korotev

from NIPR Meteorite Newsletter No. 7

Yamato 791197 (Y-791197)

Antarctica
Found: 1979 November 20
Mass: 52.4 g (1 piece)

feldspathic regolith breccia

Y-791197 is a polymict microbreccia containing clasts in a dark brown glassy matrix, similar to lunar anorthositic regolith breccias. Two or more types of clasts are observed in the thin section, such as polymineralic, monomineralic and melt clasts. Most of larger clasts are polymineralic, frequently composed of calcic plagioclase, olivine, and pyroxene; less commonly plagioclases, or plagioclases alone. Smaller clasts are mineral fragments dominantly plagioclases, with some pyroxenes and olivines, and melted lithic fragments. The clasts also show a variety of textures, including troctolitic, gabbroic, diabasic, basaltic and shock-melted glassy clasts. Most of them are more feldspathic than the HED achondrites. Y-791197 appears to be a regolith breccia with glass spherules and abundant clasts, especially feldspathic clasts, set in a dark brown glassy matrix. The PTS shows that recrystallized matrix breccias containing plagioclase fragments are the other abundant clasts. The matrix glass compositions scatter around those of the bulk rock and are similar to those of the Apollo 16 regolith breccias.

The poikilitic matrix texture is not as well developed as that observed in the Apollo 16 regolith breccias, and it is very fine-grained and can be designated them as micropoikilitic breccias. One clast (HPF) consists of dark yellowish brown to reddish brown iron-rich pyroxene, small amounts of plagioclase, fayalite, and dark mesostasis-like materials including fayalite, minor silica mineral and ilmenite. A poikilitic clast (PK1) clast has a texture with a few subround plagioclase crystals up to 0.2 mm in diameter set in a fine grayish poikilitic-like matrix. Fine-grained (up to 30 microns) olivine and pyroxene are rarely found in the matrix. The plagioclase compositions are calcic and the An contents range from 95 to 97. The Fa contents of olivine range from 36 to 49. Other small lithic clasts are: noritic and troctolitic anorthosites and shocked anorthosites. One clast (SA) consists of rounded euhedral pleonaste spinel enclosed in anorthite. This clast could be a fragment of spinel cataclasite.

Randy Says…

Yamato 791197 is a basalt-bearing (minor) feldspathic lunar meteorite. It is compositionally and texturally identical to MIL 07006, collected 2500 km away on the other side of Antarctica. The two stones may represent two different rocks blasted off the Moon from the same crater.

Yamato 791197 is the first lunar meteorite to be found in Antarctica but was not recognized to be of lunar origin until 1984 (Yanai K. and Kojima H., 1984), after ALHA 81005 was recognized.

More Information

Meteoritical Bulletin Database

Yamato 791197

Map: NIPR Location Map

References

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