Northwest Africa 8173 (NWA 8173)

Northwest Africa

Purchased: 2013 Oct

Classification: Enstatite achondrite

History: Purchased by Gary Fujihara in October 2013 from a Moroccan dealer.

Physical characteristics: The 67.2 g stone is almost entirely coated by dark fusion crust with stained whitish regions clearly visible. The cut interior consists of white regions with fresh metal and other grayish regions with orange staining from weathering of metal.

Petrography: (A. Irving and S. Kuehner, UWS) The specimen is composed mainly of stubby, prismatic grains of untwinned, pure enstatite with ~20 vol.% stained, holly-leaf shaped grains of Si-bearing kamacite, and accessory alabandite, daubreelite, alkali feldspar, silica polymorph and minor secondary calcite.

Geochemistry: Enstatite (Fs_0.0±0.0Wo_0.2-0.3, N = 4), metal (Si = 3.6 wt.%, Ni = 6.3 wt.%, Co = 0.4 wt.%, N = 3). Oxygen isotopes (K. Ziegler, UNM): analyses of acid-washed subsamples by laser fluorination gave (all in per mil) δ¹⁷O = 2.708, 2.605, 2.400, δ¹⁸O = 5.265, 5.013, 4.600, Δ¹⁷O = -0.072, -0.042, -0.029 (for a TFL slope of 0.528).

Classification: Enstatite achondrite. This specimen is unlike typical aubrites in several respects: the purity of the enstatite is remarkable (even among other known enstatite-rich meteorites), and the metal is both much more abundant and more Si-rich than in aubrites.

Specimens: 13.6 g and two polished thin sections are at UWB. The remaining material is held by Mr. G. Fujihara.