Northwest Africa 13784 (NWA 13784)

(Northwest Africa)

Purchased: 2020 Nov

Classification: Carbonaceous chondrite (CV3, anomalous)

History: The meteorite was purchased from a meteorite dealer in Morocco.

Physical characteristics: Dark-grayish rock largely covered with fusion crust.

Petrography: Carbonaceous chondrite composed of well-defined separated type I and II chondrules set in abundant fine-grained matrix dominated by ferrous Ni-free olivine. Chondrules have an apparent mean diameter of 687±579 (N=41), with a maximum diameter of 3.6 mm. Olivine is strongly compositionally zoned in most chondrules and equilibrated in the matrix. Low-Ca pyroxene is also zoned and only present in chondrules, typically concentrated in chondrule rims. Some Ca-pyroxene is present in matrix and chondrules; feldspar was not detected. Opaque phases are troilite and taenite. The section studied contains some small, mostly amoeboid-shaped CAIs predominantly composed of spinel and melilite. Sawn surfaces expose rare up to 4 mm sized CAIs with somewhat diffuse outlines.

Geochemistry: Chondrule olivine: Fa_19.6±15.6 (Fa_0.7-38.9, Cr₂O₃=0.09±0.04, n=19); low-Ca pyroxene: Fs_4.5±2.3Wo_1.2±0.4 (Fs_1.7-12.7Wo_0.7-2.5, n=21); matrix olivine: Fa_38.6±0.4 (Fa_37.9-39.5, FeO/MnO=113±9, Cr₂O₃=0.07±0.04, n=15); Ca-pyroxene: Fs_5.2±2.7Wo_46.8±1.9 (Fs_1.5-7.5Wo_42.9-48.7, n=8); oxygen isotopes (K. Ziegler, UNM): 3 acid-washed fragments analyzed by laser fluorination gave: δ¹⁸O=0.598, 0.593, 0.634; δ¹⁷O=-3.478, -3.868, -3.369; Δ¹⁷O=-3.794, -4.181, -3.703 (all per mil)

Classification: Carbonaceous chondrite (CV3, anomalous). CV chondrite based on oxygen isotopic composition and overall petrography. Absence of Cr-bearing magnetite, presence of Ni-free olivine and FeNi metal argue against CK. Matrix dominated by ferrous olivine, chondrule petrography, and dominantly small and rather rare CAIs are unusual for CV chondrites.