Northwest Africa 11961 (NWA 11961)

(Northwest Africa)

Purchased: 2018 Jan

Classification: Carbonaceous chondrite (C3, ungrouped)

History: Purchased by Greg Hupe in January 2018 from a Moroccan dealer at the Tucson Gem and Mineral Show.

Physical characteristics: Two identical brownish stones (total weight 324.3 g). Cut interior surfaces reveal light brown chondrules in a dark brown matrix containing fresh metal.

Petrography: (A. Irving and S. Kuehner, UWS; L. Garvie, ASU) Unequilibrated specimen. Composed of fairly closely packed, well formed granular chondrules (apparent diameter 840±500 µm, some with metal-rich rims) set in a fine-grained matrix (~15 vol.%, deep red-brown in thin section) containing minor secondary calcite. Orthopyroxene is rare relative to clinopyroxene, and anorthite is present both in chondrules and matrix; accessory phases include kamacite (~2 vol%) and troilite. Troilite is the only sulfide. No magnetite was observed. X-ray diffraction analysis of powdered samples detected no phyllosilicates and no crystalline clay minerals in the <1 µm size fraction. No CAI were observed in the studied thin section.

Geochemistry: Olivine (Fa_0.9-34.0, N = 3), orthopyroxene (Fs_2.1-2.3Wo_1.4-0.8, N = 3), clinopyroxene (Fs_0.9-2.7Wo_41.9-40.6, N = 2). Oxygen isotopes (K. Ziegler, UNM) Analyses of six acid-washed non homogenized subsamples by laser fluorination gave, respectively δ¹⁷O -4.928, -4.401, -3.935, -2.101, -1.850, 2.263; δ¹⁸O -2.475, -1.766, -0.748, 1.112, 1.717, 6.427 ; Δ¹⁷O -3.621, -3.469, -3.540, -2.688, -2.757, -1.130 per mil.

Classification: Carbonaceous chondrite (C3, ungrouped). Oxygen isotopic compositions for this unequilibrated chondrite plot in the same sub-TFL region as for carbonaceous chondrites, but clearly not on the trends for CR, CV, CK, CO or CM chondrites.

Specimens: 20.2 g including one polished thin section at UWB; remainder with GHupe.