Northwest Africa 7533 (NWA 7533)

(Northwest Africa)

Purchased: June 2012

Classification: Ungrouped achondrite

Petrography: (R. Hewins, MNHNP) Breccia, largest objects (~1 cm) are flattened, oval or curved fine-grained melt bodies containing crystal fragments, often with melt mantles or coatings. The melt rocks have a fine-grained subophitic to fasciculate texture (grain size 2-5 μm) and are characterized by clots with central oxides (magnetite, chromite or ilmenite) in a mass of pyroxene dendrites embedded in aureoles of plagioclase. Clasts (to ~2 mm) are dominantly crystal clasts of pyroxenes and feldspars, with magnetite and chlorapatite, small coarse-grained noritic-monzonitic fragments (>1 mm grains) made up of several of these phases, microbasalts with subophitic to granoblastic textures (grain size 1-5 μm) and melt spheres (~100 μm to >3 mm). Pyroxenes include orthopyroxene, inverted pigeonite, pigeonite (in microbasalts), and augite. Feldspars include plagioclase, anorthoclase, orthoclase and perthite. Inverted pigeonite contains 10 μm exsolution lamellae. Olivine occurs as dendrites in one melt sphere. The fine-grained interclast matrix is difficult to discern because clasts have sizes down to ~5 μm; it consists of anhedral μm-sized plagioclase with sub-μm pyroxene surrounding and embedded in it plus magnetite, often symplectitic or lacy. Rare pyrite is replaced by magnetite and hydrated or oxidized magnetite.

Geochemistry: (R. Hewins and B. Zanda, MNHNP) Orthopyroxene from Fs_21.9Wo_1.9 to Fs_49.4Wo_5.0, FeO/MnO = 34.6 (n=29), and augite from Fs_15.4Wo_40.5 to Fs_30.5Wo_39.5. Inverted pigeonite from Fs₄₀Wo₂ with Fs₂₁Wo₄₄ lamellae to Fs₄₈Wo₆ with Fs₂₁Wo₄₄ lamellae. Plagioclase An_41±8Ab_56±7Or_3±1 (n=55), perthite An_3±2Ab_22±7Or_74±9 and An_4±2Ab_89±3Or_7±2, anorthoclase An_5±2Ab_83±13Or_12±11, orthoclase An_2±0Ab_21±1Or_77±1, chromite with up to 52.4 wt% Cr₂O₃, magnetite and ilmenite with up to 5.7 wt% MgO, apatite with 1.8 to 5.4 wt% Cl. Maghemite identified by XRD. Oxygen isotopes: (J. Gattacceca, CEREGE) IR-laser fluorination/mass spectrometry gave δ¹⁸O=5.92, δ¹⁷O=3.65, Δ¹⁷O= 0.57 (all per mil).

Classification: Achondrite ungrouped. Breccia, probably paired with NWA 7034. The oxygen isotopic values are identical to those of that ungrouped achondrite breccia, and the range of mineral compositions of the two meteorites are very similar. Some of the pyroxene compositions are also very close to those of ALH 84001.

Specimens: 17 g, MNHNP, main mass with finder