Northwest Africa 3171

Algeria

Purchased 2004 February

Martian meteorite (shergottite)

A 506 g broken, greenish-gray stone, believed to have been found in western Algeria, was purchased by A. Aaronson for DGregory (DGregory).  About half of the stone is coated by black fusion crust with flow orientation, and in addition there are some small, wart-like protrusions on several sides of the stone that appear to be localized concentrations of pyroxene coated with fusion crust.  Classification and mineralogy (A. Irving and S. Kuehner, UWS; C. Herd, UAlberta): The sample consists of subequal amounts of intergrown prismatic, grayish-brown pyroxene and sparkling, glassy maskelynite.  Pyroxenes are zoned from cores of subcalcic augite (Fs_19.3Wo_33.1, FeO/MnO = 26.4)  and pigeonite (Fs_29.9Wo_12.1, FeO/MnO = 28.2) to pigeonite rims as ferroan as Fs_72.9Wo_9.8 (FeO/MnO = 39.9).  Plagioclase has been converted entirely to maskelynite and is compositionally inhomogeneous (An_41.5Or_3.7 - An_54.4Or_1.3).  Accessory phases are ulvöspinel, ilmenite, chlorapatite, Na-K-Al-Si-rich glass, silica (formerly stishovite, judging from radial cracks around some grains), merrillite, pyrrhotite and baddeleyite.  Rare barite and calcite probably are products of minor desert weathering.  Minor rusty staining around ulvöspinel grains and along thin, black fractures appears to be a complex mixture of fine-grained iron hydroxide and Si-Al-Ca-Mg-Cl-K-bearing phases, and possibly is a product of pre-terrestrial alteration (Irving et al., 2004a). This specimen is not obviously paired with any of the other four African olivine-free basaltic shergottites (Zagami, NWA 480, NWA 856 or NWA 1669).  Oxygen isotopes (T. Larson and F. Longstaffe, UWO): pyroxene d¹⁸O = +4.43 ± 0.06‰, d¹⁷O = +2.72 ± 0.0‰, D¹⁷O = +0.42 ± 0.03‰; maskelynite d¹⁸O = +4.69 ± 0.07‰, d¹⁷O = +2.82 ± 0.1‰, D¹⁷O = +0.38 ± 0.07‰. Specimens: type specimen, 22 g and four polished thin sections, UWS; main mass, DGregory.