Northwest Africa 4858

Morocco or Algeria

Find: 2004

Achondrite (howardite)

History: Purchased in July 2004 in Erfoud, Morocco, by Edwin Thompson.

Physical characteristics: A single stone weighing 329 g with a partial brown/black fusion coating.

Petrography: (A. Ruzicka and T. J. Schepker, Cascadia) In hand specimen, sample is light gray with numerous bright green crystals. It is clearly a polymict breccia with angular to subrounded clasts. These include coarse-grained diogenitic orthopyroxene clasts, which make up greater than 10% of the total material. Additionally, there are coarse-grained plagioclase mineral clasts, eucritic rock clasts that contain inverted pigeonite and plagioclase ± a silica mineral, pigeonite mineral clasts, fine-grained clasts that are interpreted to be crystallized impact melts, and some fine-grained pseudotachylite veins. Relatively rare oxide minerals are also present.

Mineral compositions and geochemistry: Most pyroxene and plagioclase grains show modest chemical zoning, but compositions between clasts differ significantly. Pyroxene types include diogentitic orthopyroxene (Wo_0.7–5 En_58–77 Fs_21–39, n = 22), non-inverted pigeonite (Wo_5–15 En_37–56 Fs_36–55, n = 9), inverted pigeonite, which contains lamellae of orthopyroxene (Wo_3–5 En_34–49 Fs_46–62, n = 9), and augite (Wo_41–43 En_31–36 Fs_21–29, n = 2). Plagioclase is fairly calcic (Ab_3–15, An_84–97, Or_0–1, n = 33).

Classification: Achondrite (howardite). Degree of shock varies from clast to clast and evidence of shock melting in a few clasts. Weathering is minimal.

Type specimens: A total of 42 g and one thin section are on deposit at Cascadia. Thompson holds the main mass.