Ungrouped Chondrites


Northwest Africa 5717

Western Sahara
Find: 2008
Chondrite (ungrouped, 3.05 subtype)

History and physical characteristics: In late 2006, just NW of Mahbas, Western Sahara, a single fresh stone was recovered that weighed 7.310 kg. Blanketed in fusion crust and showing only the slight spots of oxidation, there is one small fractured face where fusion crust is absent and the internal structure is visible. The specimen was acquired by a Moroccan meteorite and fossil dealer in Erfoud in late 2008, from whom it was purchased by Darryl Pitt in December of the same year.
Petrography (T. Bunch and J. Wittke, NAU; A. Irving UWS): An assemblage of two unique chondrule lithologies. Lithology A, the apparent host, consists of dark, tightly packed chondrules within a sparse, cataclastic matrix composed of abraded chondrule rim material (matrix = 8 vol%). Many chondrules have a thin rim of metal, sulfide, and fine-grained silicates. Chondrules are typically sub-round to sub-angular and range in size from 0.04 to 4.1 mm, mean = 0.455 mm. Matrix = 8 vol% and metal, sulfides, and metal oxides = 4 vol%. Lithology B occurs as lumpy to swirling arrays of white to gray Mg-rich chondrules that range in size from 0.05 to 3.6 mm together with a prominent subset of microchondrules (5 vol%) 0.012 to 0.050 mm in diameter. A few large (4 to 8.4 mm), irregular-shaped chondrule-like objects are present and consist mostly of enstatite. Strong compositional zoning is common in chondrule olivine and pyroxene, which are set in a mesostasis of clear, optically isotropic glass and quench crystals. Metal and sulfides occur as sub-rounded aggregates in the matrix (6 vol%).
Geochemistry: Lithology A: Olivine cores, Fa9.7-20.8 (FeO/MnO mean = 40); rims, Fa16.2-35.6 (FeO/MnO mean = 36), ferroan olivine Cr2O3 = 0.23-0.91 wt%, mean = 0.53. Lithology B: Olivine cores, Fa0.03-3.4; Cr2O3 in olivine = 0.24-0.84 wt%, mean = 0.61 wt%. Olivine rims, Fa17.2-27.5; Cr2O3 mean = 0.28 wt%. Ferroan olivine (Fa>8) Cr2O3 = 0.28-0.78 wt%, mean = 0.47 wt%. Chondrule enstatite is Fa0.04-5.8Wo0.7. Average composition of mesostasis glass is (in wt%, N = 6): Na2O = 4.65; MgO = 1.45; Al2O3 = 12.8; SiO2, 71.3; P2O5 = 0.45; K2O = 1.23; CaO = 1.38; TiO2 = 0.47; Cr2O3 = 0.04 and FeO = 5.7. Oxygen isotopes (D. Rumble, CIW): Analyses of two acid-washed whole rock samples by laser fluorination gave, respectively: lithology A, δ18 O = 4.17 and 4.47; δ17O = 2.73 and 2.90; Δ17O = 0.539 and 0.554 (all ‰); lithology B, δ18O = 3.15 and 3.68; δ17O = 1.72 and 2.01; Δ17O = 0.061 and 0.080 (all ‰).
Classification: Chondrite (ungrouped, type 3.05). Although the oxygen isotopic compositions for lithology A are similar to those of H chondrites, the very low metal content together with other characteristics are inconsistent with an H classification. Lithology B oxygen isotopic compositions are dissimilar from those of all other meteorites. The Δ17O values of lithology B are close to those of most enstatite-rich meteorites, but the mafic minerals are significantly more ferroan. Olivine Fa range and Cr2O3 contents of both lithologies are consistent with type 3.05. The weathering grade is W1/2 and the shock level is S2.
Type specimen: A total of 40 g of material and one thin section are on deposit at NAU. D. Pitt holds the main mass.

NWA 5717. Width = 23 mm. Image © T. E. Bunch, 2011