Northwest Africa 11040 (NWA 11040)

(Northwest Africa)

Purchased: 2016

Classification: HED achondrite (Eucrite, unbrecciated)

History: A complete stone was purchased from Sean Tutorow’s eBay operation by Jason Utas

Physical characteristics: One rough-surfaced, fusion crusted individual stone.

Petrography: The texture is subophitic and unbrecciated. The mineralogy is typical eucritic (pyroxene and plagioclase with minor ilmenite, silica, Cr-spinel, troilite and trace olivine), except for an unusually high proportion of pure-Fe metal, originally roughly 0.6 vol%, although roughly 1/3 of that original metal has been replaced by Fe-oxides. Plagioclase is up to 4.8 mm in length (4.8 × 0.2 mm). Metal occurs dominantly as blocky grains, typically 0.1-0.2 mm across. Manifestations of thermal annealing, such as exsolution lamellae in pyroxene, are nearly undetectable, and about 30 vol% of the rock is mesostasis, so fine-grained as to appear opaque in transmitted light. The shock class is intermediate based on undulose extinction and planar fractures in pyroxene.

Geochemistry: Pyroxenes are compositionally diverse and complex. For the most part, major-element compositions show a pattern typical of low-thermal-metamorphism eucrites (cf. Pasamonte), fanning out from a tight cluster at Fs₂₉Wo₅ (cores of larger grains) in the general direction of Fs₄₅Wo₃₀ (rims and fine-grained areas). However, in a few locales, pyroxene in fine-scale intergrowth with silica (pyroxene widths typically 1-3 microns) consists of a remarkably low-Ca variety, Fs_49-51Wo_1.0-1.5, along with a far lesser proportion of high-Ca composition (near Fs₃₉Wo₄₄). Also, curvy veinlike patches of oddly low-Ca composition (notably bright in backscattered electron images) are sprinkled within the cores of large pyroxenes. The average measured pyroxene composition (n=258) is Fs₃₇Wo₁₆ with FeO/MnO = 29±1.5. Plagioclase (n=23) is An_79-91, average An_84.6±3.4, excluding one anomalous analysis, An_98. Olivine (n=5) is Fa_69-71. Spinel (9 analyses of a single 0.6-mm grain) has Cr/(Al+Cr) ranging from 0.59-0.76. The Fe-metal has undetectable Ni (less than 0.1 wt%) and 0.15-0.23 wt% Co. INAA bulk-rock results include Na = 4.1 mg/g, Sc = 28 μg/g, Cr = 1.9 mg/g, Fe = 138 mg/g, Sm = 3.2 μg/g.

Classification: The meteorite is an unbrecciated eucrite.