Northwest Africa 13740 (NWA 13740)

Algeria

Purchased: 2020

Classification: Mesosiderite

History: Fabien Kuntz purchased the 405.3 g sample from a dealer in Algeria in 2020.

Physical characteristics: The sample is rounded in shape and has a dark reddish-brown, weathered exterior. The interior is a breccia composed of silicates and abundant metal.

Petrography: Description and classification (A. Love, App) Sample is a breccia composed of (vol%): cm-sized, angular-irregular-shaped lithic and orthopyroxene clasts (~60) set within a network of irregular-shaped, mm-sized metal grains (~40) and comminuted mixture of orthopyroxene, merrillite and plagioclase. A single subophitic clast has an average grain size 0.75 mm. Modal percentages (vol%) of silicates: orthopyroxene (68), plagioclase (28), olivine (3) and an irregular-lath-shaped silica polymorph (1). One olivine shows a thin reaction corona whereas another has been completely replaced. Matrix areas are thoroughly recrystallized and show abundant sutured grain boundaries. Additional minerals are: a silica polymorph, troilite, chromite and ilmenite.

Geochemistry: Olivine (Fa_34.1, Fe/Mn=41.3, n=1); low Ca pyroxene (Fs_32.9±3.3Wo_3.0±0.5, Fe/Mn=26.9±2.3, n=13); high Ca pyroxene (Fs_16.1±1.1Wo_41.8±1.4, Fe/Mn=21.1±2.7, n=5); plagioclase (An_95.5±1.7Or₀, n=6).

Classification: Mesosiderite (estimated class A3). Mesosiderite Class A based on texture, metal abundance, magnetic susceptibility and modal abundance (silicates) of plagioclase and Opx. Metamorphic grade 3 based on recrystallization textures within matrix and between clasts and matrix and lack of pigeonite and the partial to complete replacement of olivine.

Specimens: Fabien Kuntz holds the main mass. A slice weighing 25.3 g and a polished mount are on deposit at App.