NORTHWEST AFRICA 18601

Four relatively fresh fragments with a combined weight of 642 g were found in Mali and subsequently purchased by C. Zlimen and M. Stream. A type specimen was provided for classification to the University of New Mexico (C. Agee and T. Shisseh), and NWA 18601 was determined to be a pyroxene-phyric martian basalt.

A petrographic and geochemical description of NWA 18601 published in MetBull 115 reveals it to be primarily composed of clinopyroxene (Fs_{51.7 [±18.3]} Wo_{24.0 [±6.0]}) and maskelynite (An_{57.4 [±0.4]} Ab_{42.0 [±0.5]}), along with minor ilmenite, silica, Cl-rich apatite, symplectites, and alkali feldspar. The pyroxenes are described as strongly zoned, exhibiting cores of Ca-poor augite with Fe-rich pigeonite rims. These zoning features are consistent with rapid cooling of a more evolved magma source region, and were said to be comparable to zoning features present in the depleted, mafic, pyroxene-phyric shergottite QUE 94201.

The photo of NWA 18601 shown above is a 3.3 g thin slice demonstrating the translucency of the abundant maskelynite crystals. The photo below is a larger 8.5 g slice revealing the interior texture of this rare shergottite type, shown courtesy of Craig Zlimen. The meteorite comprises dark pyroxene crystals intermixed with shocked plagioclase transformed into maskelynite.

∗ Recent geochemical research on the martian shergottites has led to new petrogenetic models and classification schemes.  read more >>