Northwest Africa 11220 (NWA 11220)

(Northwest Africa)

Purchased: Jan 2017

Classification: Martian meteorite (basaltic breccia)

History: Purportedly recovered in Esbeta Bir Anzarane in November 2016 and purchased in Zagora, Morocco in January 2017.

Physical characteristics: A single stone with smooth black surface showing some depressions and protrusions. CT scanning indicates brecciation with heterogeneous distribution of several clast types both spheroidal and angular, as well as phenocrysts in dark matrix.

Petrography: (A. M. Krzesinska, N. V. Almeida, NHM) Imaging (SEM and micro-CT) shows a polymict breccia with a variety of lithic and igneous clasts. Matrix composed of euhedral phenocrysts and fragments of plagioclase, low-Ca pyroxene, pigeonite, augite and hedenbergite set in very fine-grained groundmass rich in Fe-oxides and pyrrhotite. Accessory minerals include: potassium feldspars, Cl-apatite, ilmenite, magnetite, Ti,Cr-magnetite and chromite. Among lithic clasts are protobreccia fragments of similar petrography and mineralogy but with their groundmass enriched in Fe-oxides. Fine-grained, devitrified impact spherules with layered textures and mantles of accreted debric and vitrophyre, fine-grained pyroxene-feldspathic glass clasts also present. Igneous clasts (basaltic, andesitic) composed of pigeonite and plagioclase with granulitic and subophitic textures. Trachyandesite clasts composed of poikilitic potassium feldspar and anorthoclase plagioclase accompanied by augitic pyroxene. Accessory Fe-Ti-P- rich clasts dominated by euhedral ilmenite, magnetite and Cl-apatite, set in feldspathic glass.

Geochemistry: Mineral compositions and geochemistry: All analyses by EPMA. Matrix: Low-Ca pyroxene, Fs_31.7±8.8Wo_2.8±0.8, Fe/Mn (afu) = 34.9±3.1 (n = 50). Pigeonite, Fs_40.8±5.7Wo_8.4±2.5, Fe/Mn (afu) = 36.8±2.6 (n = 10). Augite, Fs_22.5±4.3Wo_39.5±4.6, Fe/Mn (afu) = 32.4±4.7 (n = 35). Hedenbergite, Fs_41.1±4.1Wo_37.1±11.4, Fe/Mn (afu) = 35.1±7.7 (n = 3). Plagioclase, Or_2.6±0.7An_40.5±7.2 (n = 89). Alkali feldspar, Or_70.7±18.4An_3.9±3.0 (n = 24). Ilmenite, 3.7±1.1 wt% MgO, 1.7 ± 0.7 wt% MnO (n = 32). Magnetite with up to 2.5 wt% TiO₂, up to 1.9 wt% Cr₂O₃ and up to 0.4 wt% NiO (n = 7). Ti-magnetite, 7.4±3.5 wt% TiO₂, with up to 0.5 wt% Cr₂O₃, up to 0.4 wt% NiO (n = 16). Cr-magnetite and chromite, 25.8±9.9 wt% Cr₂O₃, up to 1.8 wt% TiO₂, up to 0.1 wt% NiO (n = 23). Cl-apatite, 0.7±0.1 wt% F, 4.9±0.3 wt% Cl (n = 62). Basaltic and andesitic clasts: Plagioclase, Or_2.4±0.8An_38.9±7.6 (n = 26). Pigeonite, Fs_33.1±4.4Wo_9.7±5.0, Fe/Mn (afu) =33.7±2.9 (n = 39). Some augite also present. Trachyandesitic and monzonitic clasts: Potassium feldspars, Or_73.9±11.3An_3.2±1.2 (n = 14). Anorthoclase, Or_6.1±1.5An_2.3±0.7 (n = 6). Plagioclase, Or_3.2±0.1An_28.3±10.9 (n = 4). Augite, Fs_18.7±5.7Wo_44.4±3.7, Fe/Mn (afu) = 30.3±9.9 (n = 14). Protobreccia clasts, impact spherules and vitrophyre clasts also present, with chemical compositions of pyroxene and plagioclase similar to matrix and phenocrysts. In general, protobreccia clasts are enriched in Fe compared with the matrix groundmass, due to the presence of sub-micron Fe-oxides.

Classification: Martian (basaltic breccia). Likely paired with NWA 7034, NWA 7533, NWA 7475, as similar in texture, mineralogy and clast types.

Specimens: 7.46 g including a 7.27 g piece and one thin section on deposit at the NHM London. M. Goff holds the main mass.