Northwest Africa 13270 (NWA 13270)

Algeria

Find: 2019 Feb 7

Classification: Ordinary chondrite (LL3.5)

History: Found by nomads in Algeria on 7 Feb 2019, purchased by Stephen Amara from Mustapha Hnini in Tindouf on 10 Oct 2019.

Physical characteristics: Single stone with dark-brown fusion crust.

Petrography: (D. Sheikh, FSU; A. Love, App) Sample is a chondrite composed of well-defined chondrules (Av. 815±50 μm, range 250-1802 μm, n=37) and a few dark, fine-grained inclusions (<5mm) set in an opaque matrix. Devitrified mesostasis is present in most chondrules while isotropic glassy mesostasis is relatively rare. Many chondrules are armored with sulfides and contain rounded globules of FeNi metal. (A. Love, App) Optical CL: Sample exhibits bright and distinct CL. Sample is dominated by chondrules with olivine displaying dull red luminescence (A5) and blue to blue-purple mesostasis (B2, B3), however the sample does contain 5% A2 chondrules. Chondrules with bright red luminescence lack the prominant CL zoning seen in lower petrologic types. Chondrule rims and matrix show dim, dark red and blue CL (>type 3.4, <type 4).

Geochemistry: (D. Sheikh, FSU) Olivine (Fa_18.2±11.1, range Fa_1.6-45.0, CaO wt% =0.11±0.07, n=42), Cr₂O₃ in ferroan olivine (0.06±0.04 wt%, range 0.01-0.26, n=32), Low-Ca Pyroxene (Fs_9.6±6.2 Wo_0.5±0.4, range Fs_1.0-23.4 Wo_0.2-1.5, n=21).

Classification: Ordinary Chondrite (LL3.5) LL group is due to average chondrule diameter and magnetic susceptibility. Subtype 3.5 is due to numerous reasons: 1) While devitrified mesostasis may be more dominant in the chondrules, there is still primary isotropic glassy mesostasis contained within some chondrules as evident by the 5% of A2 chondrules (Dehart et al. 1992). 2) The Cr₂O₃ values and standard deviations for ferroan chondrule olivines plot in the 3.2 field for UOCs. 2) The measured fayalite and CaO values for chondrule olivines indicate a petrologic type <3.8 (Huss et al. 2006). Overall, the presence of primary glassy mesostasis and ferroan olivine containing low subtype Cr₂O₃ values in a small percentage of chondrules (5%) indicates that the peak metamorphic temperature or duration during thermal metamorphism of the sample was not enough to fully devitrify the primary glassy chondrule mesostasis throughout all of the chondrules, one of the observed features for subtype 3.6 (Dehart et al. 1992). Therefore, this sample is estimated to have a subtype of 3.5.

Specimens: 7.65 grams at UCLA; main mass with Stephen Amara.