Northwest Africa 13276 (NWA 13276)

(Northwest Africa)

Purchased: 2019 Dec

Classification: Martian meteorite (Shergottite)

History: Four separate but identical-appearing stones were purchased in December 2019 independently by Aziz Habibi (2200 g, 772 g) and Rachid Chaoui (390g, 290 g) from dealers in Mali and Mauritania. Subsequently the 2200 g and 772 g stones were acquired by Darryl Pitt, and the 290 g stone was acquired by Dr. David Gregory.

Physical characteristics: The specimens lack fusion crust. Fresh interiors are relatively coarse grained and exhibit deep brown olivine grains accompanied by pale green pyroxene and sparkling interstitial maskelynite.

Petrography: (A. Irving, UWS and P. Carpenter, WUSL) Poikilitic texture (mean grainsize 0.7 mm). Composed predominantly of compositionally-zoned pyroxene oikocrysts (with domains of pigeonite and subcalcic augite) enclosing chadacrysts of shock-darkened olivine (with limited compositional zoning), accompanied by lath-like maskelynite and accessory Ti-poor chromite, Ti-chromite, ilmenite, merrillite, pyrrhotite, pentlandite and baddeleyite. Melt inclusions are present within olivine grains. Small shock melt pockets are present and minor secondary barite was observed.

Geochemistry: Olivine (Fa_33.1-37.1, FeO/MnO = 46-53, N = 14), zoned olivine (core Fa_40.4, rim Fa_35.2, FeO/MnO = 51-53), pigeonite cores (Fs_21.5-22.8Wo_3.4-7.6, FeO/MnO = 27-32, N = 7), subcalcic augite cores (Fs_15.5-18.5Wo_27.6-33.1, FeO/MnO = 25-27, N = 4), pigeonite rims (Fs_23.0-30.0Wo_8.6-14.0, FeO/MnO = 27-30, N = 7), subcalcic augite rims (Fs_15.6-16.4Wo_32.5-36.7, FeO/MnO = 23-26, N = 3), maskelynite (An_57.5-59.5Or_1.6-0.9, N = 6).

Classification: Shergottite (poikilitic).

Specimens: 21.5 g including one polished thin section at UWB; remainder with Mr. A. Habibi, Mr. R. Chaoui and Dr. D. Gregory.