Northwest Africa 13444 (NWA 13444)

(Northwest Africa)

Purchased: 2017 oct 29

Classification: Carbonaceous chondrite (CK4/5)

History: Purchased at Munich mineral show 2017 by Marc Jost from Aziz Habibi

Physical characteristics: Dark brownish black, wind-eroded, pitted surface, no fusion crust. Some protruding CAIs.

Petrography: Chondrules have a mean apparent diameter of 0.96 mm (N=48), maximum diameter 1.8 mm. Matrix (~70%) is significantly recrystallized with feldspar grain size of 10-40 μm. Magnetite is by far the most abundant opaque mineral, occurring as fine dust in the silicates, as single grains up to 50 μm and composite grains up to 300 μm. Sulfides only occur as inclusions in magnetite, but common iron hydroxides indicate the former presence iron sulfides. The meteorite is weakly shocked (S2). Almost complete weathering of sulfides (excluding inclusions in magnetite), olivines partly stained with Fe-hydroxides and partial transformation of magnetite to maghemite indicate strong weathering. Some veins of secondary calcite. The meteorite is brecciated based on some clearly visible clasts up to 3 mm.

Geochemistry: Olivine Fa_32.1±1.5 (range Fa_30.0-35.5, N = 18); low-Ca pyroxene Fs_12.9±6.8Wo_1.5±1.1 (N = 12); high-Ca pyroxene Fs_17.1±0.9 (N = 4); feldspar An_34.9±13.0Or_1.7±1.2 (N =5). Silicates in the matrix are well equilibrated, large chondrules show Fe-poor, unequilibrated central parts. Magnetite is Cr-rich with Cr₂O₃ 3.9±0.3 wt% (N = 11).

Classification: Based on abundance and recrystallization of matrix and mineral composition including Cr-content of magnetite this is a CK chondrite. Based on feldspar grain size and partially incomplete equilibration the petrographic grade is estimated as 4/5. Shock grade S2, strong weathering.

Specimens: 20.675 g and one polished thin section at NMBE, main mass: Marc Jost.