Northwest Africa 10918 (NWA 10918)

(Northwest Africa)

Purchased: 2013

Classification: Carbonaceous chondrite (CV3)

History: A meteorite was purchased by Mike Miller in 2013 at the Tucson Gem and Mineral Show and subsequently acquired by the Center for Meteorite Studies, ASU.

Physical characteristics: A single 61.86 g angular stone with shiny black (possibly sand-blasted) exterior; chondrules and CAIs visible on the surface. Sawn surface is orange stained, and shows an abundance of chondrules and CAIs set in a dark matrix.

Petrography: (E. Dunham and J. Zalesky, ASU) In a 2 × 3 cm section, there are ~80 chondrules (average 1.2 mm n=20, largest ~ 3 mm), and four CAIs (to ~ 1 cm in diameter), set in a dark, fine-grained matrix that accounts for ~32% of the section. Chondrules are predominantly POP, and a few BO, PP, and olivine fragments. Some chondrules are surrounded by a rim of FeS and weathered metal. Mesostasis rare within chondrules, but where present consists of anorthite (An₉₃). Most chondrules have an irregular outline, but the boundary between them and matrix is distinct. The thin section contains a Type B CAI composed of melilite, pyroxene, and spinel surrounded by rim of pyroxene and hibonite, and a compact Type A (CTA) CAI dominated by coarse melilite with small perovskite blebs and surrounded by a <50 μm rim including pyroxene, and a very fine-grained object (possibly a CAI). The shock grade is low and weathering high.

Geochemistry: The compositional range for olivine in chondrules is Fa_0.7±0.3, 0.3-1.4 (Cr₂O₃ = 0.08-0.51 wt.%). Olivine phenocrysts from nine POP chondrules is Fa_0.3-1, and olivine phenocrysts from one intact PO chondrule is Fa_0.8-1.4. The POP chondrules contain pyroxene with a compositional range Fs_46±6.8Wo_72.2±24.0 (Fa_0.2-23Wo_38-97). The Type B CAI is composed of Al-rich melilite and pyroxene Fs_3.6±2.3Wo_64.2±8.3 (Fs_0.5-5Wo_47-67). The CTA CAI is primarily composed of melilite Ak~5-15 with a rim of Ca- Ti- rich pyroxene Fs_0.6-1.8Wo_62-64.

Classification: Carbonaceous chondrite CV3, high weathering, low shock.

Specimens: One polished thin section, two thick sections, the main mass is held by ASU