Basaltic Shergottite


NWA 3171


A 506 g broken, khaki-gray, ellipsoidal stone, believed to have been found  at an unspecified site in western Algeria. One shield-like face of the stone is coated by fresh, black fusion crust with flow lines radiating away from a small dimple-like central depression, and in addition there are some small, black, wart-like protrusions on other sides of the stone.

Classification and mineralogy (A. Irving and S. Kuehner, UWS; C. Herd, UAlberta): The sample consists mainly of about equal amounts of intergrown prismatic, grayish-brown pyroxene and sparkling, glassy maskelynite. 

Very thin, subparallel, black glass veins and irregularly-distributed small pockets of vesicular, dark brown glass occur within the interior of the specimen.  The latter evidently are responsible for the wart-like, exterior protrusions, and probably were produced by shock-induced melting. Pyroxenes are zoned from cores of subcalcic augite (Fs19.3Wo33.1, FeO/MnO

= 26.4) and pigeonite (Fs29.9Wo12.1,, FeO/MnO = 28.2) to pigeonite rims as ferroan as Fs72.9Wo9.8 (FeO/MnO = 39.9).  Plagioclase has been converted entirely to maskelynite and is compositionally inhomogeneous (An41.5Or3.7 - An54.4Or1.3).  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.  Coexisting Fe-Ti oxide compositions imply oxygen fugacities 1.3 to 1.4 log units below QFM at temperatures of 761-782 degrees C.  Rare barite and calcite probably are products of minor desert weathering.  Minor rusty staining around ulvöspinel grains and along thin, black shock veinlets appears to be

a complex mixture of fine-grained iron hydroxide and Si-Al-Ca-Mg-Cl-K-bearing phases.  This specimen is not obviously paired with any of the other four African olivine-free basaltic shergottites (Zagami, NWA 480/1460, NWA 856 or NWA 1669).  Oxygen isotopes (T. Larson and F. Longstaffe, UWO):