Northwest Africa 13791 (NWA 13791)

(Northwest Africa)

Purchased: 2020

Classification: Ungrouped achondrite

Petrography: (K. Metzler, IfP) Equigranular igneous rock with some vesicles. It consists of ~85% mafic components (olivine and low-Ca pyroxene with Ca-rich pyroxene rims) and ~15 vol% fine-grained interstitial material. The typical grain size is ~300-1000 µm. Metal and sulfide beads up to 800 µm are completely oxidized by terrestrial weathering.

Geochemistry: Mineral compositions and geochemistry: Olivine grains show igneous chemical zoning, with increasing Fe concentrations from core to rim. Mean olivine composition is Fa_20.5±3.2 (Fa_16.4-29.5; n=17); Fe/Mn (at.): 50±12. Mean low-Ca pyroxene composition is Fs_20.1±0.9Wo_2.7±0.3 (Fs_18.1-21.4Wo_2.2-3.1; n=17); Fe/Mn (at.): 29±10. The mean composition of Ca-rich pyroxene is Fs_18.6±2.1Wo_28.1±3.9 (Fs_15.0-22.3Wo_21.7-34.1; n=8); Fe/Mn (at.): 28±7. Mean bulk composition of fine-grained interstitial material (EDX analyses; area ~50x100 µm; wt%): SiO₂ 64.2; Al₂O₃ 17.1; Na₂O 9.4; FeO 4.2; CaO 3.2; MgO 0.5; K₂O 0.8; TiO₂ 0.7. Based on mineral chemistry, this sample has affinities to L or H ordinary chondrites.

Classification: Ungrouped achondrite: The combination of mineral compositions does not fit to any known achondrite group, although it exhibits affinities to H or L ordinary chondrites.