Northwest Africa 2924

Algeria

Find: 2005

Mesosiderite

History: Over 180 kg (several thousand pieces) havebeen recovered from a small strewn field in Algeria. The largest known pieceweighs 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 andpartially recrystallized subophitic to granular basalts that enclose larger (upto 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 basaltscontain unusually calcic plagioclase (An_99.3) with little FeO(<0.04 wt%); highly twinned pigeonite is compositionally similar to the muchlarger clast pigeonite (Fs₃₂Wo_5.7), but with very littleCr₂O₃ and Al₂O₃. Merrillite is alsopresent. Metal blebs (Ni = 6.6 wt%) are irregular in shape and typically <2mm in size with the exception of metal aggregates and rare, centimeter-sizemetal nuggets with included pigeonite.

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

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