The only CR3 !
Northwest Africa 12474 (NWA 12474)
Classification: Carbonaceous chondrite (CR3)
Physical characteristics: Dark stone. Cut surface reveals dark interior with chondrules.
Petrography: Chondrite with well-defined chondrules set in an abundant fine-grained matrix (55 vol% from point counting, n=609). Average apparent chondrule diameter 650±310 μm (n=40). Strong weathering. Opaque minerals are FeNi metal, magnetite, troilite, often forming composite opaque grains. The chondrules are layered with metal/sulfide rims; opaques are also found blebs up to 300 µm in size, and some metal blebs occur at the surfaces of chondrules. XRD does not show evidence for the presence of phyllosicates. Based on the structural order of the polyaromatic matter assessed by Raman spectroscopy (L. Bonal, IPAG), this meteorite appears slightly metamorphosed, less than Bishunpur (type 3.1). The Raman spectroscopy parameters of interest are FWHM-D (cm-1)=180.3±8.7, ID/IG=0.96±0.04, N=7 (see Bonal et al. 2016, figure 6). Terrestrial weathering is strong with alteration veins crosscutting the meteorite.
Geochemistry: Olivine Fa1.9±0.8, PMD=35%, Fa0.5-2.9, Cr2O3=0.4±0.2wt%, (n=5). Low-Ca pyroxene Fs2.4±0.7Wo1.2±0.4, range Fs1.3-3.3Wo0.6-1.8 (n=7). Plagioclase An86.4±2.7Ab13.5±2.8.0Or0.0±0.1 (n=2). Magnetic susceptibility log χ (× 10-9 m3/kg) = 4.69.
Classification: CR3. CR group based on petrography (chondrule size, metal abundance), and geochemistry. Type 3 based on the absence of phyllosilicates; subtype is ~3.1 based on Raman spectroscopy .
Specimens: 29 g and a polished section at CEREGE. Main mass with Kuntz.