Northwest Africa 1863 (NWA 1863)

(Northwest Africa)

Purchased: 2019

Classification: HED achondrite (Eucrite)

History: The meteorite was exported from Morocco for purchase by N. Gessler.

Physical characteristics: A subangular single stone, 60% covered by brown stained matte black grainy non-crazed fusion crust. Broken and cut faces show two textures: a "very light gray N7" matrix with <1 mm grains, and a moderate brown 5YR 4/4 matrix with <4 mm grains.

Petrography: About half of the thin section is a fragmental, but probably monomict, breccia, with typical grain size about 0.10 mm. The other half is essentially unbrecciated, showing a coarse subophitic texture with pyroxene (showing typical eucritic fine-scale exsolution) up to 5 mm, lathy plagioclase up to 2 mm, and silica laths up to 3.5 mm. The grain size of the unbrecciated portion is not consistently that coarse, but the texture is consistently subophitic. The boundary between the brecciated and unbrecciated lithologies is generally quite sharp.

Geochemistry: Mineral-composition data indicate no significant difference between the brecciated and unbrecciated components. Exsolution is coarse enough for EPMA (13 analyses) to resolve low-Ca from high-Ca pyroxene, which cluster near Fs_61.2Wo_4.2 and Fs_31.0Wo_40.8, respectively; averaging Fs_51.7Wo_15.7, with FeO/Mno (wt ratio) = 33.6. Plagioclase (11 analyses) ranges from An_74.9 to An_90.3 and averages An_87.7.

Classification: Eucrite, brecciated, probably monomict

Specimens: Main mass with Gessler, type specimen, including multiple thin sections, at UCLA.