Northwest Africa 7635 (NWA 7635)

(Northwest Africa)

Purchased: 2012 May

Classification: Martian meteorite (Shergottite)

History: Purchased near Dakhla, Morocco in May 2012 by Ali and Mohammed Hmani.

Physical characteristics: A single black, partly fusion crusted stone (195.8 g), containing prominent lath-shaped, glassy maskelynite phenocrysts (some in clusters) in a fine-grained black matrix.

Petrography: (A. Irving and S. Kuehner, UWS) Porphyritic texture. Phenocrysts of intermediate plagioclase (as blocky laths, some in groups, completely converted to maskelynite), ferroan olivine, subcalcic augite and Ti-free magnetite are set in a much finer grained, quenched matrix dominated by clinopyroxene (zoned from augite to ferropigeonite) in parallel growth with plagioclase. Accessory pyrrhotite and rare ilmenite are present. No phosphates were found.

Geochemistry: Olivine (Fa_60.5-60.6; FeO/MnO = 43-49), subcalcic augite (Fs_28.5Wo_32.3; FeO/MnO = 38; TiO₂ = 0.20 wt.%; Cr₂O₃ = 0.93 wt.%), ferropigeonite Fs_60.6-77.2Wo_17.6-21.8; FeO/MnO = 41-48). Bulk composition (G. Chen, UAb): Analysis by ICP-MS of powder ground in agate from a 1.1 g interior slice gave (in ppm) Ni 7.7, Rb 0.40, Sr 69.2, Zr 9.8, Ba 9.9, La 0.19, Ce 0.83, Nd 1.38, Sm 0.85, Eu 0.33, Gd 1.65, Yb 1.99, Lu 0.30. Oxygen isotopes (K. Ziegler, UNM): three acid washed subsamples analyzed by laser fluorination gave, respectively (all per mil), δ¹⁷O = 2.536, 2.521, 2.513; δ¹⁸O = 4.241, 4.220, 4.217; Δ¹⁷O = 0.297, 0.294, 0.287 (for a TFL slope of 0.528).

Classification: Martian (shergottite, olivine-plagioclase-phyric). This specimen is distinct from other shergottites in containing phenocrysts of very ferroan olivine and maskelynite.

Specimens: 21.2 g of material and 2 polished thin sections are on deposit at UWB. The main mass was held by Hmani  and subsequently was sold to an anonymous collector.