Northwest Africa 8708 (NWA 8708)

(Northwest Africa)

Purchased: 2005 Oct 1

Classification: Ordinary chondrite (L6)

History: A single unclassified NWA stone was purchased by Edwin Thompson at the Munich show in October 2005.

Physical characteristics: Prior to cutting (with alcohol), the sample consisted of a completely fusion coated faceted individual forming a roughly triangular shape. Fusion coating in places is black and shiny; elsewhere brownish or covered by patches of caliche, with numerous shrinkage cracks. Cut faces are medium brownish gray, with distinct lighter colored chondrules visible. Surfaces also show abundant irregular bright white objects, as well as numerous pits/pores.

Petrography: (M. Hutson and A. Ruzicka, Cascadia) In reflected light, most of the thin section lacks visible metal and contains moderately weathered troilite, with pits filled with a complex mixture of calcium suflate, iron oxide/hydroxide, and a material that is dominantly silica. These secondary minerals appear to be the bright white objects observed in hand specimen. Elaborate layers of weathering product ring the pits. Although most plagioclase feldspar grains are <50 μm across, 100 μm grains are relatively common and chondrules and grains grade into each other, consistent with a type 6 chondrite. Overall, roughly 60-70% of the opaques in the section have been replaced, indicative of a W3 weathering grade. Olivine grains are abundant and relatively heavily fractured, but have uniform to very slightly undulose extinction. It is estimated that slightly less than 25% of the grains show slight undulose extinction, indicative of an S1 shock stage.

Geochemistry: Olivine (Fa_25.1±1.1, N=15) and low-Ca pyroxene (Fs_21.0±1.1Wo_1.7±0.6, N=12). Iron values may be slightly high due to extensive FeO weathering veins which cut through all grains, making it difficult to obtain clean analyses.

Classification: Based on texture and mineral chemistry this stone is a fairly heavily weathered L6 chondrite.

Specimens: 39.2 g as well as polished thin section and the corresponding stub are on deposit at Cascadia. The main mass is held by Patrick Thompson of ET Meteorites.