Tiffa 022        19°29’10.5’’N, 10°35’29.5’’E

Agadez, Niger

Purchased: 2021 Jan

Classification: Mesosiderite

History: Purchased from a moroccan mineral dealer. Found in Niger in September 2019.

Petrography: (K. Metzler, IfP) Achondritic breccia, consisting of monomineralic (low-Ca pyroxene) and polymineralic (low-Ca pyroxene + plagioclase) clasts up to 2 mm, set in small amounts of finer grained, recrystallized matrix. Ca-rich pyroxene with thin exsolution lamellae is poikilitically enclosed in low-Ca pyroxene. Modal amount of plagioclase >10 vol%. Rare olivine grains up to 1.3 mm occur, surrounded by reaction zones consisting of low-Ca pyroxene and Cr-spinel. Severe metal oxidation with crisscrossed oxide veins throughout the sample. Accessories are a silica polymorph, merrillite, and troilite. Metal: sample has 29 vol% metal, and nearly all metal in this sample has been converted to oxidation products.

Geochemistry: Mineral compositions and geochemistry: Mean low-Ca pyroxene composition Fs_29.6±3.8 Wo_2.8±1.0 (Fs_22.5-37.4 Wo_1.6-4.4); mean Fe/Mn (at.): 27.8; n=20. Mean Ca-rich pyroxene composition Fs_19.2±1.4 Wo_38.4±0.8 (Fs_17.5-20.6 Wo_37.3-39.3); mean Fe/Mn (at): 18.8; n=5. Mean olivine composition: Fa_38.8±0.4 (Fa_38.2-39.1); mean Fe/Mn (at.): 35.7; n=6. Mean plagioclase composition An_90.6±3.1 Or_0.0±0.1 (An_84.9-94.3 Or_0.0-0.3); n=14. Metal: Only one metal grain (core of a large oxidized grain) was detected in the investigated thin section (kamacite; ~7 at% Ni).

Classification: Mesosiderite, based on texture, mineralogical composition, and mineral chemistry. Proposed subtype B2 (Plagioclase 10-25%, moderately recrystallized matrix).