Northwest Africa 7304 (NWA 7304)

(Northwest Africa)

Purchased: Sept 2009

Classification: Ureilite

History: Samples were donated to Cascadia by Mr. Fred Olsen in June, 2010.

Physical characteristics: The rock has a blunt nosecone shape. Fusion crust with radial flow lines emanating from the nose covers ~70% of the specimen. The interior is a relatively uniform dark-gray color.

Petrography: (A.Ruzicka and M. Hutson, Cascadia) An olivine-rich rock (>75%) with low-Ca pyroxene, graphite laths to 1 mm, and Fe-oxide and hydroxide minerals (hematite, goethite, and common hematite-magnetite mixtures). Ca-sulfate is rarely present in veins. Olivine-rich and pyroxene-rich regions are ~1 mm in diameter. Olivine-rich areas are partly recrystallized (shock stage S6) and have recrystallized domains showing one dominant lineation, but are mainly composed of small (<10 μm diameter) subhedral olivine grains with interstices between olivine (<3 μm wide) filled by low-Ca pyroxene, high-Ca pyroxene, and chromite. Interstitial regions between millimeter-sized olivine areas are rich in pyroxene, symplectic Fe-oxide and hydroxide phases, and graphite, the latter partly replaced by Fe-oxide minerals.

Geochemistry: (A. Ruzicka and M. Hutson, Cascadia) Fine-grained, olivine-rich areas far from graphite contain relatively ferrous olivine (Fa_22.3±0.3, Fe/Mn = 49±11 at., n=28) and low-Ca pyroxene (Wo_4.4±1.5 Fs_21.2±1.0, n=10), whereas closer to graphite, olivine (Fa_8.1±2.5, Fe/Mn = 17±4 at.%, n=11) and low-Ca pyroxene (Fs_12.4±1.8Wo_2.8±0.9, n=9) are more magnesian.

Classification: Achondrite (ureilite). Partly shock-recrystallized and melted. Metal is largely replaced by Fe-oxide weathering products but silicates were largely unaffected by weathering.

Specimens: 22.5 g, one polished butt, and one thin section are on deposit at Cascadia. Mr. McKenzie holds the main mass.