Northwest Africa 2924

Algeria

Find: 2005

Mesosiderite

History: Over 180 kg (several thousand pieces) have been recovered from a small strewn field in Algeria. The largest known piece weighs about 5000 g; most are small pieces that weigh less than 200 g.

Petrography and Geochemistry: (T. Bunch and J. Wittke, NAU) Fine-grained (<0.5 mm) matrix consists of brecciated and partially recrystallized subophitic to granular basalts that enclose larger (up to 2.5 mm) subrounded to subangular olivine (Fa₂₈), pigeonite (Fs₃₁Wo_5.4; FeO/MnO = 25.7; Cr₂O₃ = 0.87, Al₂O₃ = 1.28 [both wt%]) that rarely shows twinning. These cataclastic basalts contain unusually calcic plagioclase (An_99.3) with little FeO (<0.04 wt%); highly twinned pigeonite is compositionally similar to the much larger clast pigeonite (Fs₃₂Wo_5.7), but with very little Cr₂O₃ and Al₂O₃. Merrillite is also present. Metal blebs (Ni = 6.6 wt%) are irregular in shape and typically <2 mm in size with the exception of metal aggregates and rare, centimeter-size metal nuggets with included pigeonite.

Classification: Mesosiderite; low shock, variable degree of weathering dependent on the specimen size.

Specimens: An 85 g type specimen is on deposit at NAU. MFarmer holds a 16 kg sample.