Northwest Africa 12692 (NWA 12692)

Mali

Purchased: March 2019

Classification: Ordinary chondrite (LL3.00)

History: Reportedly found in 2012 by nomads in Mali. Purchased in Morocco, March 2019.

Physical characteristics: Sixteen identically appearing stones with masses ranging from 2-157 g. Exterior surface is desert-weathered with some caliche, no fusion crust. A saw-cut surface reveals many densely packed chondrules, the largest of which is 7 mm in diameter. The largest chondrule (or clast) is irregular in shape and has vesicles. The matrix is black and a few sulfide/metal grains are visible.

Petrography: (C. Agee, UNM) Microprobe examination of a polished mount shows numerous unequilibrated chondrules, apparent mean diameter 600±575 μm (n=47), many with porphyritic, igneous-zoned olivines and pyroxenes, most with glass or mesostasis. Abundant fine-grained matrix is present throughout. Minor metal.

Geochemistry: (C. Agee, UNM) Chondrule olivine Fa_12.4±6.5, Fe/Mn=41±20, Cr₂O₃=0.40±0.11 wt%, n=31; chondrule low-Ca pyroxene Fs_3.1±1.8Wo_0.7±0.8, Fe/Mn=9±3 n=10. Ferroan chondrules with pyroxenes are rare: most pyroxenes analyzed were in Mg-rich type I chondrules, hence the low Fs mean value.

Classification: Ordinary chondrite (LL3.00). LL based on chondrule size and low abundance of metal. Type 3.00 based on ferroan olivine mean Cr₂O₃ content and sigma from Grossman and Brearley (2005) and by the presence of S-rich opaque matrix. Comparison of Cr₂O₃ in ferroan chondrule olivines with those done using the same microprobe and conditions at UNM gave: NWA 7731 (L3.00) Cr₂O₃=0.43±0.11 wt%, n=98; NWA 8276 (L3.00) Cr₂O₃=0.42±0.10 wt%; Semarkona (LL3.00) Cr₂O₃=0.41±0.09 wt%, n=45.

Specimens: 25 g on deposit at CSFK, probe mount at UNM, ZKereszty holds the main mass.