Northwest Africa 5230                                                                                

Northwest Africa

Find: October 2007

Achondrite (polymict eucrite)

History: Purchased by Space Jewels Switzerland in October 2007 from a Moroccan dealer in Switzerland.

Physical characteristics: A single stone (909 g) composed of abundant white to dark gray clasts (up to 5 mm), and less abundant limpid, pale bluish to colorless maskelynite crystals (up to 1.4 mm), in a finer grained, medium gray matrix.

Petrography (A. Irving and S. Kuehner, UWS): Polymict breccia composed of mineral fragments and lithic clasts in a finer matrix of the same phases.  Mineral fragments include compositionally variable low-Ca pyroxene (mostly unexsolved pigeonite and some orthopyroxene), completely maskelynitized calcic plagioclase, exsolved pigeonite (orthopyroxene with augite lamellae), unexsolved subcalcic ferriaugite (rimmed by fayalite), olivine, silica polymorph, chromite, ilmenite, Ni free metal, troilite and rare zircon.  Lithic clasts include both cumulate and basaltic eucrites, intergrowths of troilite+orthopyroxene+silica polymorph, and a rare mesosiderite clast (composed of ~50 vol.% metal with orthopyroxene, calcic plagioclase and chromite).

Geochemistry: Pigeonite (Fs_31.1Wo_11.0, FeO/MnO = 29.6), some with rims of ferroan low-Ca pyroxene (Fs_55.9Wo_3.1, FeO/MnO = 30.7); diogenitic low-Ca pyroxene (Fs_26.2Wo_3.5, FeO/MnO = 28.6); plagioclase (maskelynite, An_76.5-95.1Or_1.7-0.2).

Classification:  Achondrite (polymict eucrite).  This specimen is paired with NWA 4883, and contains distinctive large plagioclase grains completely transformed to maskelynite; the presence of clasts of mesosiderite is unusual among eucrites.  The sparseness of diogenitic debris makes this specimen a polymict eucrite breccia rather than a howardite.

Specimens: A total of 22.25 g of sample and one polished thin section are on deposit at UWS.  The main mass is held by Space Jewels Switzerland.