Northwest Africa 4425

Algeria

Find: May 2006

Carbonaceous chondrite (CK3)

History: A stone was purchased by P. Thomas in May 2006.

Physical characteristics: A complete, ventifacted stone weighing 579 g, without fusion crust. It has a brownish color indicating significant terrestrial alteration.

Petrography: (M. Bourot-Denise, MNHN) Chondrules are mm-sized. Most CAIs are also visible to the naked eye. The type specimen contains a CAI 1.5 cm in diameter rimmed with spinel and diopside. Chondrules are generally porphyritic with wide rims, and contain magnetite beads. Large opaque nodules of magnetite and sulfide are present in the matrix. Matrix is either crystallized or fine, with abundant small magnetite crystals. BSE images indicate that this chondrite is not completely equilibrated; olivines and pyroxene in porphyritic chondrules are zoned. In the type specimen, there is an angular clast, 2 cm in diameter, containing small, porphyritic pyroxene chondrules (100- 500 µm); chondrules and well-crystallized matrix are completely equilibrated; magnetite is finely scattered through the clast and does not make up nodules as in the host; the interface between the clast and the host is highlighted by small, euhedral magnetites.

Geochemistry: Olivine (Fa_13.6±12.6; range Fa_0.54-34.1), low-Ca pyroxene (Fs_16.8±11.0; range Fs_2.8-30.7), magnetite (Cr₂O₃ = 2.6-3.7, MgO = 0.87-1.15 [both wt%]) all in the CK3 domain. Pyroxene in the angular clast: Fs_23.35±0.86. Magnetic susceptibility (J. Gattacceca, CEREGE) log χ = 4.6 10^-9 m³/kg.

Classification: Carbonaceous chondrite (CK3, estimated subtype 3.8); minimal shock and moderate weathering.

Type specimens: A total of 32.8 g of sample (1 sawn fragment) and two polished mounts are on deposit at MNHN. Thomas holds the main mass.

Submitted by: M. Bourot-Denise, MNHN.