Northwest Africa 13411 (NWA 13411)

Western Sahara, Morocco

Purchased: 2018

Classification: OC5-an

History: The 190.96 and 73.03 g samples were found near Adrar, Algeria, by a meteorite prospector in 2018. Dave Lehman and his partner acquired the samples from the finder.

Physical characteristics: Samples are irregularly shaped and brown. Interior of sample is orangish brown and contains abundant chondrules and weathered metal and sulfides.

Petrography: Description and Classification (A. Love, App) Sample has a chondritic texture composed of numerous close-packed equilibrated chondrules set within a comminuted matrix of the chondritic lithology. This matrix is crosscut by ubiquitous FeNiOH veinlets. When viewed using backscattered electron imaging, the olivines and pyroxene have almost identical gray-scale colors. The sample is composed of BO, PO (dominant), PP (clinoenstatite is rare but present) and rare Al-rich chondrules with an average apparent diameter of 395±219 µm (n=131). Some pyroxenes are zoned from enstatite cores to augite rims. The sample contains secondary plagioclase with an average grainsize of 29 µm (n=15). Additional minerals are: chromite, K-rich feldspar, apatite, kamacite, taenite, troilite and gypsum. Despite a comprehensive search for reduced sulfide and phosphide phases commonly associated with enstatite and forsterite chondrites, none were found in this sample.

Geochemistry: (A. Love, App) Olivine (Fa_8.35±0.15, Fe/Mn=16.97±0.58, n=18), low-Ca pyroxene (Fs_8.47±0.26Wo_1.09±0.32, Fe/Mn=10.78±0.39, n=18); high-Ca pyroxene rims (Fs_3.44±0.08Wo_46.93±2.71, n=4). plagioclase of variable composition (Ab_18.17-83.41Or_0.66-12.88, n=6); FeNi metal (6.71±0.21wt% Ni and 1.09±0.02wt% Co, n=9). Oxygen Isotopes: (Karen Ziegler, UNM) analyses of acid-washed subsamples by laser fluorination gave (all per mill): δ¹⁷O=4.199, 3.277, 3.836, 3.592, 3.666, 3.102; δ¹⁸O=5.891, 5.520, 6.526, 5.464, 6.433, 5.240; Δ¹⁷O=1.089, 0.362, 0.390, 0.707, 0.269, 0.335.

Classification: Ordinary chondrite (OC5-an, C-S2, W3) Based on chondritic texture and presence of weathered and unweathered metal, this sample has similarities with ordinary chondrites. Based on mineral compositions and oxygen isotopes, this is an anomalous ordinary chondrite. Equilibrated compositions and grainsize of secondary plagioclase indicate this sample corresponds to petrologic type 5. Oxygen isotope compositions plot at a distance from the slope 1 line for equilibrated ordinary chondrites. Mean Δ¹⁷O composition is below the standard range for unequilibrated H chondrites and the standard deviation of the Δ¹⁷O is higher than that recorded in the entire range of unequilibrated ordinary chondrites. Co concentrations in kamacite fall between values measured in L and LL chondrites (Rubin, 1990).

Specimens: Dave Lehman and a partner hold the main mass. Two slices totaling 25.08g and a polished thin section and mount are on deposit at App.