Northwest Africa 5743 (NWA 5743)

(Northwest Africa)

Purchased: 2009 Feb

Classification: HED achondrite (howardite)

History: Purchased in February 2009 by Adam Hupé from a Moroccan dealer at the Tucson Gem and Mineral Show.

Physical characteristics: A single, uncrusted stone (216 g) containing several varieties of metal-free clasts and distinctive, dark-colored, relatively metal-rich clasts (to 1.5 cm).

Petrography: (A. Irving and S. Kuehner, UWS): Heterogeneous fragmental breccia composed of lithic clasts and various mineral fragments (plagioclase, orthopyroxene, exsolved pigeonite, chromite, taenite). Harzburgitic diogenite clasts (~30 vol%) consist of subequal amounts of olivine and low-Ca pyroxene with accessory chromite and troilite. Metal-rich ultramafic clasts (~30 vol%) consist of strongly-zoned olivine and low-Ca pyroxene with 10-15 vol.% metal (kamacite with minor taenite). Sporadic basaltic eucrite clasts consist of prismatic exsolved pigeonite grains, plagioclase laths, clinopyroxene, silica polymorph, chromite, ilmenite, zircon, troilite and Ni-free kamacite.

Geochemistry: Metal-rich ultramafic clast - olivine (core Fa_8.7, FeO/MnO = 59.9, rim Fa_35.0, FeO/MnO = 42.9), low-Ca pyroxene (Fs_23.2-30.7Wo_1.8-2.1, FeO/MnO = 31.6-33.9); harzburgitic diogenite clast - olivine (Fa_28.0, FeO/MnO = 52.9), low-Ca pyroxene (Fs_22.5Wo_2.0, FeO/MnO = 29.3); basaltic eucrite clast - low-Ca pyroxene host (Fs_61.3Wo_3.0, FeO/MnO = 29.6), Ca-pyroxene (Fs_32.6Wo_38.2, FeO/MnO = 32.4), plagioclase (An_87.2Or_0.6). Discrete plagioclase mineral clasts are An_86.3-90.5Or_0.5-0.3. Oxygen isotopes (D. Rumble, CIW): duplicate analyses of acid-washed samples by laser fluorination gave the following results for different components. Basaltic eucrite clast: δ¹⁸O = 3.41, 3.41; δ¹⁷O = 1.53, 1.52; Δ¹⁷O = -0.261, -0.273 per mil. Metal-rich ultramafic clast: δ¹⁸O = 3.55, 3.24; δ¹⁷O = 1.60, 1.44; Δ¹⁷O = -0.266, -0.261 per mil.

Classification: Achondrite (howardite). This breccia is unusual in containing diverse material related to harzburgitic diogenite, cumulate eucrite, basaltic eucrite, and metal-rich ultramafic lithologies. Little, if any, olivine-free diogenitic material appears to be present. The metal-rich ultramafic clasts have oxygen isotope compositions typical for eucrites and diogenites, yet their constituent olivine grains have unusually magnesian cores.

Specimens: 20.1 g and one polished thin section of a complete slice are on deposit at UWS. AHupé holds the main mass.