Northwest Africa 5492 (NWA 5492)

(Northwest Africa)

Purchased: 2008

Classification: Ungrouped chondrite

History: A 593 g, partially crusted stone, was purchased in Morocco by A. Aaronson in 2008 and sold to M. Farmer and J. Strope in Tucson, Arizona, in February, 2009.

Petrography: (T. Bunch and J. Wittke, NAU; M. Weisberg, KCCU): This stone is a matrix-poor mixture of (given in order of abundance) sub-mm to cm-size "barred" silicate and melt clasts (52 vol %), small Mg-rich cryptocrystalline, radial, and glassy chondrules with minor PP, barred, and POP chondrules and fragments (25 vol %) and rounded to sub-angular metal clumps (23 vol %), and grains of schreibersite, pyrrhotite, and FeS with daubreelite exsolution lamellae. In addition, another section contains a 5 mm chondrite clast with chondrules and fragments <500 μm in size and large angular clasts consisting of a fine-grained intergrowth of enstatite, metal and sulfide. Some clasts are dominated by quench-crystallized enstatite, Al-diopside, forsterite and feldspathic mesostasis together with metal enclaves. Barred clasts show shock-melted/vesiculated margins and consist of forsterite, enstatite, diopside, feldspathic glass, and metal. Metal lumps typically contain arcuate textures of tiny pyroxenes, forsterite, and FeS grains interstitial to composite metal grains. Weathering grade is W2; shock level variable, S2 to S5.

Geochemistry: Barred silicate clasts: olivine, Fa_0.5; low-Ca pyroxene, Fs_1.2Wo_3.1 (Al₂O₃ = 8.4 wt. %); diopside, Fs_1.4Wo_47.1 (Al₂O₃ = 7.3 wt %). Smaller cryptocrystalline chondrules: olivine, Fa_{0.2 -0.7}; low-Ca pyroxene, Fs_0.1-0.7Wo_0.5. mesostasis glass, Na₂O = 2.3 - 2.83 wt % (N= 7). Metal Ni = 7.1 wt %. Oxygen isotopes: (D. Rumble, CIW) average for 4 analyses of barred silicate clasts (standard deviation): δ¹⁷O = -2.13, δ¹⁸O = 0.29 and Δ¹⁷O = -2.28 per mil. Average of 6 analyses for the smaller reduced chondrules and fragments (standard deviation): δ¹⁷O = 2.93, δ¹⁸O = 4.79, Δ¹⁷O = 0.43 per mil.

Classification: Chondrite (ungrouped), type 3. Petrologic characteristics together with the oxygen isotopic data suggest a relationship to CB chondrites for the large barred clasts, whereas the smaller reduced chondrules and fragments have oxygen isotope signatures that suggest a possible relationship to the E chondrites.

Specimens: A total of 20.7 g and one thin section are on deposit at NAU* [Ed: reported lost 21 June 2024]. Main mass,  MFarmer and Strope.