Northwest Africa 4832

Northwest Africa

Find: 2006

Achondrite (aubrite)

History and physical characteristics: Hundreds of platey fragments, the largest weighing 33.1 g, were found in northwest Africa and purchased by A. Aaronson in 2007. The total known weight is 268 g. Fragments tend to break along the preferred orientation plane of constituent enstatite grains. Most of the dark brown pieces are moderately to heavily weathered, a few show small, fresh metal nuggets.

Petrography: (T. Bunch and J. Wittke, NAU) composed mostly of fine-to medium-grained (0.3 to 1.2 mm), euhedral to subhedral cumulate enstatite together with interstitial metal, schreibersite, graphite, FeS, a silica phase present as round inclusions in FeS, daubreelite, brezinaite and very fine-grained intercumulus plagioclase. Much of the metal is oxidized. The stone is unbrecciated with a low shock level.

Mineral chemistry: Enstatite is essentially Fe-free (En_99.6Fs_0.1Wo_0.3). Kamacite contains 4.5 to 5.6 wt% Si and FeS contains 2.6 wt% Ti. The rare mineral brezinaite contains S, 45.2 wt%; Cr, 36.9 to 46.6 wt%; Fe, 18.7 to 9.0 wt% and 1.1 wt% Mn.

Classification: Achondrite (aubrite)

Type specimen: A total of 20.6 g is on deposit at NAU. Aaronson is the main mass holder.