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Northwest Africa 7915: A New approved LL5 chondrite from Morocco

Brawata M., Kryza R., Jakubowski T., Przylibski T. A., Ćwiąkalski J., Łuszczek K.

Meteorites

2014

Vol. 3, No. 1-2

45--58

A 415 g single meteorite was purchased in 2010 by T. Jakubowski from a dealer in Morocco. The meteorite was isometric in shape, ca. 8 cm in size, with distinct regmaglypts on the original ablated surface, and covered mostly in primary crust with one broken surface. The weight of the sample studied was 69 g. The meteorite is composed of several types of chondrules including porphyritic-Ol-Px, barred-Ol, radial-Px, granular and cryptocrystalline with distinct and diffused (not sharp) boundaries, and opaque grains and aggregates, enclosed in a very fine-grained matrix. The average compositions of minerals are: olivine (both in chondrules and matrix) − Fo70.4Fa29.1Te0.5, pyroxenes, represented by Mg-Fe (Ca-poor) orthopyroxene (and minor clinopyroxene?) − En73.9Fs24.1Wo2.0, feldspars (small in the matrix and in barred chondrules), with An12-37, and Or~3-4, taenite − Fe 70.80, Ni 25.50 and Co 1.67 wt. %, troilite − Fe0.98S1.00, chromite (Fe2+ 0.96Mg0.12Mn0.01Zn0.01) (Cr1.52Al0.23Fe3+ 0.02Ti0.10Si0.02)O4; altered accessory minerals including apatite and iron-rich secondary phases have also been identified and analyzed. The meteorite is of petrologic type 5, as evidenced by the observed recrystallization of the matrix, relatively good preservation of the chondrule structures, homogeneous composition of olivine and pyroxene, and the presence of only secondary small feldspar grains. The shock stage, S2, is based on the presence of undulatory extinction and irregular fractures in olivine crystals. The weathering grade, W3, is confirmed by the observation that kamacite is totally altered into secondary iron phases, whereas Nirich taenite, and troilite are only partly weathered. The specimen shows many bulk- and mineral-chemical parameters corresponding, mostly, to the LL chondrite group (e.g., Fe/SiO2 0.49, SiO2/MgO 1.62, Fa in olivine 29.05). However, concentrations of several other elements, including REE, are not fully consistent with the average values for the LL ordinary chondrites. Apparently, the parent body of the studied NWA 7915 meteorite was depleted in Dy, Tm, and Yb, compared to typical LL-type ordinary chondrite parent bodies. Also, relatively high concentrations of other elements, including Ba and Sr, have been measured, which may result from terrestrial weathering in hot desert conditions. The meteorite has been classified as LL5 ordinary chondrite, S2, W3, and registered in the Meteoritical Society database as NWA 7915. The type specimen is deposited in the Mineralogical Museum of the University of Wrocław.