Northwest Africa 13969 (NWA 13969)

(Northwest Africa)

Find: 2017

Classification: Carbonaceous chondrite (C2, ungrouped)

History: Several partially fusion-crusted stones weighing 102 g acquired by Ruben Garcia and Bob Cuchiara from Adam Aaronson at the 2017 Tucson Gem and Mineral show. 42 g was acquired by the Center for Meteorite Studies at ASU.

Physical characteristics: (L. Garvie, L. McCann, J. Miech, ASU) The largest fragment (27.3 g) is partly covered with ~0.3-1.0 mm in thick ropy fusion crust. The stones were noteworthy for being exceptionally light when picked up. The density of the 27.3 g stone is 1.56 g/cm3 (determined using glass bead methods with 100-µm sized beads). Microporosity estimated at 32% from BSE images. Total porosity estimated 46-51%, based on bulk grain densities of Murray and Murchison. The interior of the stones is dark gray-green with an abundance of small (<1 mm) chondrules and mineral fragments. Under the binocular microscope, the broken surfaces appear highly vesicular.

Petrography: (L. McCann, J. Miech, M. Hernandez, L. Garvie, ASU) A powder XRD pattern shows intense reflections for serpentine, tochilinite, olivine, magnetite/maghemite, calcite, and pentlandite. Low-magnification BSE images of the polished mount show abundant chondrules and chondrule fragments with fine-grained rims loosely packed together. The high porosity is evident as dark (BSE images) areas between the chondrules. Abundant porphyritic chondrules and rarer barred olivine chondrules, set within a serpentine-tochilinite dominated matrix. Chondrules have mean diameter of 268 µm (range=69-1277 µm, n=103). Chondrules primarily Type I (Mg # >90). Rare CAIs contain spinel, Al-rich pyroxene, hibonite, melilite, and perovskite and average ~135 µm (range 86-200 µm, n = 8). CAIs are mantled by thick fine-grained rims. Rare Mg-rich carbonates (average 126 µm in diameter, n = 9) are present. Rare metallic phases include a 10 µm kamacite grain within an olivine chondrule. Sparsely distributed 40-50 µm sulfides present in the form of pentlandite, pyrrhotite, and troilite. BSE images show euhedral fibrous tochilinite (to ~5 µm in length) within the pore spaces.

Geochemistry: Oxygen isotopes (K. Ziegler, UNM): 3 fragments analyzed by laser fluorination gave δ¹⁸O= -0.682, -3.272, -3.395; δ¹⁷O= -5.761, -7.509, -8.149; Δ¹⁷O=-5.400, -5.781, -6.357 (linearized, all per mil, TFL slope=0.528). Microprobe (L. McCann, J. Miech, M. Hernandez, A. Wittman, ASU):Two distinct populations of olivine, one with Fa_2±1, n=58, Range: 0.5-6.4 and one with Fa_34±11 n=19, Range: 17.2-57.7. Minor element phases in olivine CaO up to 1.8 wt%, Cr₂O₃ up to 0.6 wt%, NiO up to 0.1 wt%, Al₂O₃ up to 0.2 wt%. Pyroxene composition is En_65-98, Fs_1-34, Wo_1-4 (n = 12).

Classification: C2 ungrouped. The stone has a type 2 petrographic type as evidenced by the serpentine dominated matrix . It contains abundant and relatively small chondrules and CAI. While mineralogically similar to CM chondrites, the oxygen isotopic compositions fall below CCAM at the lower end of the CV-CK field.

Specimens: 42 g at ASU.