NWA 16476
NWA 16476
Found Sahara, 2022
Carbonaceous Chondrite
C2-Ungrouped
Pending on the Meteoritical Bulletin
Petrography:Description and classification (A. Love, App) Sample shows an unbrecciated
chondritic texture composed of: unequilibrated chondrules (average apparent diameter 190μm
n=207), chondrule fragments, mm-sized CAI’s and AOA’s; TCI’s and clastic matrix. 1 vol% FeNi
metal occurs as rounded droplets within chondrules along rims and within matrix. Chondrules
show rare isotropic glassy, commonly turbid and altered mesostasis composed of
phyllosilicates. Chondrite components in some regions (Area A) are surrounded by dust
mantles, show abundant TCI ’s set within a matrix dominated by hydrated (brighter in BSE
image-Fig. 2) phases. Some chondrules have contain mesostasis composed of a mixture of
phyllosilicates. Chondrules, fragments and refractory inclusion make up ~ 21 vol% of sample.
Hydrous phyllosilicates were detected by transmission IR spectroscopy (T.J. Prestgard, Univ.
Grenoble Alpes). Additional minerals are: troilite, chromite and apatite.
Geochemistry:Geochemistry of sample was measured using the JEOL ITS300 SEM with Oxford
XMax EDS in the Dewel Microscopy Lab at Appalachian State University. An accelerating voltage
of 20kV was used to analyze 3 spots per grain. Matrix analytical totals were measure at EPS-UTK
using the Cameca SX-100 microprobe using a 15kV and 30nA beam with a 15μm spot size.
Compositions are equilibrated.
(A. Love, App) Area A: Olivine (Fa26.7±26.8, Cr2O3 in Fa>2=0.34±0.13, Fe/Mn=61.9±39.1,
n=12); low Ca-pyroxene (Fs6.8±7.8W2.1±1.2, Fe/Mn=17.4±12.0 n=5). Matrix analytical total –
89.3±3.1wt%
Area B: Olivine (Fa16.7±21.7, Cr2O3 in Fa>2=0.36±0.09, Fe/Mn=53.4±46.2, n=12); low Ca
pyroxene (Fs2.3±1.6Wo2.8±2.8, Fe/Mn=13.7±5.0, n=8). Matrix analytical total – 94.5±3.3wt%.
Oxygen Isotopes:Oxygen isotopes (D. Ibarra, Brown U): analyses of acid-washed subsamples by
laser fluorination gave, respectively, δ17O =-10.022, -8.589; δ18O =-7.341, -8.397; Δ17O =-6.146,
-4.137 (all per mil and normalized to San Carlos which was analyzed simultaneously). Oxygen
isotopes plot along the CCAM but are distinct from CO and CM chondrites. Oxygen isotope
compositions have more negative d18O compositions than CO chondrite despite the presence
of hydrated phases.
Classification:Carbonaceous chondrite (C2 ungrouped C-S1 W1) Chondrule diameters and
magnetic susceptibility suggest sample is CO chondrite, however hydrated phases are
anomalous for CO chondrites. Presence of phyllosilicates confirmed by low analytical totals
within matrix and transmission IR spectroscopy. Oxygen isotope composition indicates sample
is distinct from CO chondrites but also distinct from most C2 ungrouped chondrites that occupy
the O isotope compositional region between CM and CO chondrites. Variably hydrated regions
are anomalous for CO chondrites.
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