Northwest Africa 12328 (NWA 12328)

(Northwest Africa)

Purchased: 2017

Classification: Carbonaceous chondrite (CM1)

History: Ruben Garcia and Bob Cucchiara purchased eight similar-looking, partially fusion-crusted meteorite fragments weighting 43.2 g from Adam Aaronson at the 2017 Denver Gem show.

Physical characteristics: The largest fragments 20.2 g and 10.6 g have one surface with well-developed, ropy fusion crust. Pieces without fusion crust are gray and show slight wind abrasion. Fresh broken surfaces exhibit an earthy fracture, are black and studded with tiny crystalline white flecks and sparse protruding chondrules and nodules. Stones are very soft and can be easily crushed by hand to a fine powder.

Petrography: (L. Garvie, ASU) Powder x-ray diffraction of the bulk meteorite shows a pattern dominated by serpentine, with low-intensity reflections for sulfides (mackinawite, pentlandite), magnetite, bassanite, carbonates (ankerite, calcite), and very weak reflections possibly from olivine. The 001 serpentine is a well-resolved doublet with maxima at 0.735 and 0.724 nm. Powder XRD from each of the fragments comprising the 43.2 g shows the double-peaked 001 serpentine reflection. Powder XRD patterns from several nodules and chondrules shows patterns dominated by the 0.735 nm clay. One chondrule also shows a three-component 0.7 nm reflection and weak 1.4 nm peak, suggesting the presence of minor chlorite. A polished mount of a 2 × 1.5 cm fragment shows two well-defined chondrules (1 mm and 0.9 m diameter), smaller (<0.5 mm) chondrules and dark nodules, and sparse sulfide grains (<100 μm) set in a dark matrix with abundant fine-grained sulfide. Analysis of a 2 × 2 mm region of the section shows that chondrules, relict chondrules, and clay-rich nodules comprise 41 areal% of the section. Chondrules and relict chondrules have a mean diameter of 331 μm (n=25). An Mg x-ray map shows magnesium "hot-spots" in a few chondrules corresponding to relict olivine grains. BSE images show that the olivine is deeply corroded and partially replaced by clays. Olivine represents ~1 areal% of the section, consistent with the weak peaks in the powder XRD profile. No CAIs were found. Some of the fine-grained sulfide may be partially replaced by terrestrial oxides.

Geochemistry: (L. Garvie, ASU) WDX of a few relict olivine grains. BO chondrule with olivine bars largely replaced by clay. One bar has Fa_1.1. A 200 μm grain in matrix, Fa_18.3. The 1 mm chondrule contains a 0.5 mm olivine grain, Fa_0.5. A 100 μm olivine grain in 300 μm-sized chondrule, Fa_1.4.

Classification: Carbonaceous chondrite (CM1). The chondrules to matrix ratio and average diameter of the chondrules are consistent with a CM chondrite. The predominance of serpentine and only minor amounts of olivine (~1 areal%), which show extensive alteration to clays, argue for a petrologic grade 1 carbonaceous chondrite.

Specimens: 8.7 g and one polished mount at ASU.