Northwest Africa 6871
A single extremely hard, brown stone weighing 119 g and almost impossible to cut. Small adamantine crystals of diamond were extracted from the stone by Mr. Catterton.
Coarse-grained aggregate of olivine and orthopyroxene with finer grained interstitial regions containing opaque material, calcite and limonite. Some thin calcite veinlets cross-cut the specimen. Both olivine and pyroxene are completely recrystallized to aggregates of myriad tiny, polygonal subgrains.
Olivine cores Fa19.4-20.0 (CaO=0.3 wt.%, Cr2O3=0.7 wt.%), rims Fa10.5; orthopyroxene Fs12.6-13.7Wo3.7-3.4.
Achondrite (ureilite). The very high shock experienced by this specimen (causing complete recrystallization of silicates and evidently generation of significant amounts of microdiamond) is unusual among ureilites.