Sample No.: ALHA77257

Location: Allan Hills

Field No.: Y78010301

Weight (gms): 1995.7

Meteorite Type: Achondrite (ureilite)

Physical Description:

Approximately one-half of the sample's surface is rounded and mostly covered with fusion crust ~2 mm thick. This crust is dark brown with areas of reddish (iron oxide?) staining. Small areas of the fusion crust have been plucked revealing crystalline structure. The remaining two-thirds of the sample consists of three planes which are fracture surfaces. These surfaces are crystalline, rough on a mm scale, and show no evidence of fusion crust. The fracture surfaces are dark brownish-black and are moderately weathered with small patches of what appears to be iron oxide stain. Crystalline grains with well-developed crystal faces set in a black, fine-grained matrix make up the broken surface. Some grains are covered with a dark stain and others are milky white to clear. The small, white to clear anhedral grains are aligned in a strip across the broken surface (N & T views) and part of the fusion crust. The sample is not a complete stone. Its approximate measurements are 16x11x9.5 cm. The small white anhedral grains are visible on the surface of the meteorite in the field photos. One grain was removed and placed in an aluminum cup and allowed to warm. The grain retained its original form after an hour of warm temperature.

Petrographic Description: Brian Mason

This meteorite is an achondrite (ureilite), consisting almost entirely of anhedral to subhedral olivine (~80%) and pyroxene (~15%); it is fairly coarse-grained, with olivine grains up to 4 min, pyroxene to 3 mm. The olivine grains show undulose extinction. The pyroxene shows coarse polysynthetic twinning. Grain boundaries are marked by a concentration of carbonaceous material; trace amounts of troilite and nickel-iron, partly altered to limonite, occur along grain boundaries. Microprobe analyses show olivine of variable compositions (Fa9-23, average Fa13) and with unusually high Ca (0.2-0.3%) and Cr (0.3-0.4%) contents. The pyroxene is a low-calcium clinopyroxene with composition average Wo7En81Fs12. The meteorite is extremely resistant to cutting and polishing, which probably indicates the presence of diamond, as in other ureilites.