ERG ATOUILA 001

Achondrite, ungrouped
Albitite
(trachyte/syenite igneous rock type)

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Found 2020
22° 29' 24" N., 4° 33' 6" W.

Two wind-abraded stones weighing 553 g and 240 g were found within a km of each other in the Sahara Desert region of northern Mali (Google Earth image). Both stones were purchased by Mohamed Aid who sent a sample to the University of New Mexico for analysis and classification (M. Spilde, A. Ross, C. Agee, and K. Ziegler). A paired individual weighing ~30 g was also recovered (photo courtesy of A. Habibi). The meteorite was designated Erg Atouila 001 and classified as an ungrouped achondrite having the characteristics of an albitite (95 vol% albitic alkali feldspar).

In addition to the predominant albite, Erg Atouila 001 contains ~3 vol% clinopyroxene (Ti-, Cr-, Na-rich augite) along with accessory phases including ferroan olivine, K-feldspar, fluorapatite, troilite, very low Ni Fe-metal (some oxidized), and ilmenite (Agee et al., 2022 #2442). The feldspars in Erg Atouila 001 form an alkali feldspar solid solution with albite and orthoclase endmembers (see a feldspar ternary diagram). In a comparison of volcanic rocks based on chemistry and plotted on a total alkali vs. silica diagram, Erg Atouila 001 is the first analyzed meteorite that falls within the trachyte field (see TAS diagram below). Alternatively, the relatively coarse texture of Erg Atouila 001 could be indicative of an igneous plutonic or shallow intrusive origin, in which case its mineralogical composition would be consistent with a syenite, or more specifically, a sodic felsic syenite as assessed by Irving et al. (2022 #2059) (see QAPF diagram below).

Total Alkali vs. Silica (TAS) Diagram
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Diagram credit: Agee et al., 53rd LPSC, #2442 (2022)
'Erg Atouila 001: Unique Albitite Achondrite Meteorite'

QAPF Streckeisen Ternary Diagram
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Diagram credit: Antonio Ciccolella

Erg Atouila 001 has an oxygen isotope composition and Fe/Mg and Fe/Mn ratios that are similar to those of the acapulcoite–lodranite clan meteorites. On the oxygen three-isotope diagram below, Irving et al. (2022) demonstrate that Erg Atouila 001 plots precisely along the least-squares regression line originally established by Clayton and Mayeda (1996) from data points for the acapulcoite–lodranite clan, and which has been further populated by many accepted and proposed (e.g., Choteau, NWA 468) related meteorites. However, such apparent O-isotopic correspondence has been shown in previous cases to be merely coincidental through the use of coupled ε54Cr vs. Δ17O diagrams. Until Cr isotopes and other data are obtained for Erg Atouila 001, no reliable conclusions about parent body origin can be made.

Oxygen Isotopes for Erg Atouila 001 and Acapulcoite–Lodranite Clan
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Diagram credit: Irving et al., 53rd LPSC, #2059 (2022)
'Ultrasodic, Albite-Rich Syenitic Achondrite Erg Atouila 001: A Highly Evolved
Felsic Igneous Differentiate From The Acapulcoite-Lodranite Parent Body'

A diligent search of the area surrounding the Erg Atouila 001 find site has resulted in the recovery of five additional stones located in the near vicinity—between 40 and 900 meters (44 yards to half a mile) away. Analyses of these five stones, weighing together 1442 g, were completed by Irving and Carpenter (2022 #6370), and the data confirm that the component minerals in the new stones are chemically close to those in Erg Atouila 001 and close to acapulcoites. However, there are some obvious mineralogical differences between them, as well as some differences in both the phases observed and the modal abundances. Nevertheless, It is easy to infer that these five stones were once incorporated together with EA001 in a single heterogeneous meteoroid that was earlier ejected from their parent body. It is now arguably even more likely that these meteorites originated on the acapulcoite–lodranite asteroid.

The photo of Erg Atouila 001 shown above is a 3.75 g slice that was cut from the piece shown in the excellent high-res photographs below, courtesy of Phil Schmitt. The natural surface lacks fusion crust and shows a light-orange desert patina, while the cut face exhibits coarse-textured grains of light pink to white albitic feldspar intermixed with dark green to black pyroxene and sparse Fe(Ni)-metal.

standby for erg atouila 001 photo
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standby for erg atouila 001 photo

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Photos courtesy of Phil Schmitt