Northwest Africa 10519 (NWA 10519)

Mauritania

Purchased: 2015 Sept

Classification: Enstatite achondrite (ungrouped)

History: Purchased by Michael Farmer from a meteorite dealer at the Denver Gem and Mineral Show, September 2015. The stone was reportedly found in Mauritania.

Physical characteristics: Single, deeply regmaglypted stone weighing 18.197 kg, with remnant fusion crust. Fusion crust on some edges of the stone is worn away, exposing metal. A 21 × 14 cm slab displays a brecciated texture dominated by sub-angular to rounded dark-green to tan metal-silicate clasts, separated by an anastomosing network of metal veins. Recognizable clasts up to 10 cm. Clasts range from blocky, dark-greenish and metal poor (<2 area %) to sub-rounded with ~20 area % metal. The dark blocky clasts commonly have a lighter colored greenish rim (to 1 mm thick). Overall the weathering grade is low, W1, although a few areas near the edge of the sample show W2. Shock stage is low, troilite is single crystal and metal lacks Neumann bands.

Petrography: (D. Schrader and L. Garvie, ASU) Electron microprobe analysis shows the silicates are enstatite, diopside, and albitic plagioclase, with minor silica. No olivine was observed. Metal is commonly associated with troilite, alabandite, daubrelite, and minor graphite and schreibersite. Areas of metal, to 2 cm across, show well-developed Widmanstätten pattern, with variable band-width with an average of 0.5 mm. Individual kamacite lamellae typically short, stubby and swollen, with rounded ends, and locally forms 120° triple grain boundaries with adjacent lamellae. Metal shows well-developed dark etching and comb plessite. Metal grain boundaries are decorated with submicron precipitates. Alabandite is present as grains up to 1 mm wide around large metal segregations and grains within troilite. Daubrelite is present as lamellae within troilite. Metal in clasts typically shows grains with holly-leaf to amoeboid outlines.

Geochemistry: (D. Schrader, ASU) Enstatite Fs_0.6±0.3Wo_1.2±0.4, Fe/Mn=5±2, n=13; diopside Fs_0.7±0.2Wo_46.6±1.0, Fe/Mn=6±5, n=7; plagioclase An_21.6±0.2Ab_76.4±0.3Or_1.9±0.1, n=3; and silica. Opaque minerals observed include kamacite (wt.%; Fe 92.6±0.4, Ni 6.6±0.2, Co 0.4±0.0, Si <0.01, P 0.1±0.0; n = 16), taenite (wt.%; Fe 71.2±7.0, Ni 27.7±6.4, Co 0.1±0.0, Si <0.01), P <0.02; n = 2), troilite (wt.%; Fe 62.7±0.1, S 36.6±0.2, Ti 0.17±0.03, Cr 0.37±0.07; n = 4), alabandite, daubréelite, and schreibersite.

Classification: Ungrouped enstatite achondrite. The meteorite is texturally and chemically similar to the EL6 chondrite Blithfield, compared to data in Rubin (1984). However, this meteorite is distinguished from enstatite meteorites by the lack of Si in metal (below 0.01 wt.%) and the low Ti content of troilite (0.17±0.03 wt.%).

Specimens: A sample of 562 g and a polished mount is at ASU. Main mass of 5.5 kg with Boudreaux. Remainder with MFarmer.