Northwest Africa 5721 (NWA 5721)

Morocco

Found: 2007

Classification: HED achondrite (Eucrite)

History: A small dark stone was found in SW Morocco (Western Sahara) in 2007 and purchased in Erfoud by an anonymous buyer.

Physical characteristics: A single fresh stone (67.5 g) with a dark gray, partly translucent fusion crust, and containing numerous millimeter-sized vesicles of pre-terrestrial origin.

Petrography: (T. Bunch and J. Wittke, NAU; A. Irving, UWS): The texture is dominated by long (<12 mm), bladed, dark pyroxenes; some swallow-tailed shapes are present and form lamellar textures with plagioclase, in addition to ophitic to sub ophitic textures. Smaller sub-textures include variolitic and plumose with interstitial poikilitic residuum that contains tiny pyroxenes set in masses of a low birefringent silica phase, iron sulfide (probably troilite), fayalite, metal, ilmenite, and felsic glasses. Pigeonite and augite textures are complex, with large changes in grainsize over short distances. Numerous irregularly-shaped, smooth-walled, empty vesicles range in size from 0.2 to 2.5 mm.

Geochemistry: Pigeonite (Fs_53.2-68.1Wo_13.5-8.0, FeO/MnO = 35), augite (Fs_39.4-44.4Wo_32.2-36.7, FeO/MnO = 27), olivine (Fa_85.7, FeO/MnO = 48), plagioclase (An_82.3-88.0). Oxygen isotopes (D. Rumble, CIW): acid-washed material analyzed in replicate by laser fluorination gave, respectively, δ¹⁸O = 2.93, 2.99; δ¹⁷O = 1.26, 1.32; Δ¹⁷O = -0.281, -0.254 per mil. This composition plots at the lower bound of the field for eucrites. Bulk composition (by ICP-AES and ICPMS, J.-A. Barrat, UBrest): in wt. %, TiO₂ 0.58, Al₂O₃ 12.49, FeO 19.62, MnO 0.55, MgO 4.07, CaO 10.46, Na₂O 0.43, P₂O₅ 0.21; in ppm, La 1.64, Ce 4.24, Nd 3.21, Sm 1.05, Eu 0.47, Gd 1.47, Yb 1.30, Lu 0.20, Rb 0.7, Sr 68, Ba 20, Ni 1.2, Ga 2.8. The chondrite-normalized REE pattern is essentially flat at ~5.4x chondrites, with a small positive Eu anomaly.

Classification: Achondrite (eucrite). Although this specimen has mineral compositions and oxygen isotopic composition similar to those of typical eucrites, it differs from them in the following significant ways: presence of abundant vesicles, low bulk MgO content and very high FeO/MgO ratio (both beyond values for basaltic eucrites, and reflected in the high ferrosilite contents of some constituent pyroxenes), high Ga/Al ratio (~1.75 times higher than in eucrites), rare earth element abundances ~1.65 times lower than those in basaltic eucrites, and much higher abundances of P and Rb than in eucrites. Shock level and weathering grade are both low.

Specimens: 13.8 g and one polished thin section are on deposit at NAU. The main mass is held by an anonymous owner.