ALHA77005 Petrographic Description

Antarctic Meteorite Newsletter, Volume 4, Number 1
February 1981

Sample No.:	ALHA77005
Field No.:	77122928
Weight (gms):	482.5
Meteorite Type:	Achondrite (unique)
Location:	Allan Hills

Physical Description: The specimen is well rounded on all faces except the south face, which is a partially broken surface. A well developed, dark fusion crust randomly covers approximately 5% of the meteorite's surface in small, thin patches. Parts of the stone are covered by a vitreous crust that possibly is the lower portion of the dark, outer fusion crust, which has been physically abraded. Some partially melted crystals are in contact with this glassy rind. The specimen's dimensions are ~9.5 cm x 7.5 cm x 5.25 cm.

The specimen is well­indurated and difficult to chip. This would be a good meteorite for the study of fusion crust. The outer dark crust has apparently been removed by physical weathering. There is little evidence that the glassy rind has been affected by chemical weathering. The thin section chip may not be representative of the entire specimen.

On the cut surface the sample appears to be fresh. Variations in color of sawed surface suggest heterogeneities on a centimeter scale. Voids up to 2 mm in diameter are present on the sawed surface. Some of these voids show crystal structures on the peripheral surface, but they do not appear to extend into the cavity.

Petrographic Descriptions: Brian Mason
This meteorite is an achondrite with the following modal composition (volume percent): olivine, ~55; pyroxene ~35; maskelynite, ~8; opaques (mostly chromite, a little ilmenite, trace amounts of troilite and nickel­iron), ~2. Olivine occurs as somewhat rounded anhedral to subhedral grains up to 2 mm long, and has an unusal pale brown color; microprobe analyses show a mean composition of Fa₂₈. Pyroxene occurs as colorless prismatic crystals up to 6 mm long, often poikilitically enclosing olivine; some crystals show coarse polysynthetic twinning; the composition is somewhat variable averaging Wo₅ Fs₂₃En₇₂. Maskelynite is present as laths interstitial to olivine and pyroxene; it has labradorite composition An₅₃ and contains 0.2­0.3% K₂O. The meteorite has been severely shocked, as is shown by the presence of maskelynite, undulose extinction in the pyroxene, and occasional areas of apparent shock melting. No signs of weathering were observed.

Meteorite ALHA77005 is a unique achondrite. The olivine is comparable in composition to that in chassignites, the pyroxene to that in diogenites, and the bulk composition will thus be intermediate between these two classes of achondrites. However, these classes are almost plagioclase­free, and maskelynite of similar composition is known only from the Shergotty and Zagami achondrites; these meteorites are quite different from ALHA77005 in consisting largely of calcium­rich clinopyroxene and containing no olivine.