Figure 1. Optical transmitted light image taken in cross-polarized light.
Sodium-rich mafic (Mg-Fe-rich) igneous rocks are rare, even on planet Earth. Among meteorites, one of the few examples is Graves Nunataks 06128/06129 (paired stones found in Antarctica and thought to be related to brachinites). Sodium-rich plagioclase feldspar (albite) also occurs in aubrites and other enstatite achondrites, but only as a minor component. In May 2020 a very different sort of plutonic igneous achondrite was found as multiple specimens (total weight at least 32 kilograms) in the Erg Chech sand sea located in far southern Algeria near the mutual borders with Mali and Mauritania. This unique unbrecciated achondrite is composed predominantly of a gabbroic lithology (composed mainly of pigeonite and sodic plagioclase), but it is striking because of the sporadic presence of large (up to 9 cm) megacrysts (more accurately xenocrysts) of various sorts of green to yellow-green pyroxene (augite, orthopyroxene, pigeonite), which exhibit embayed shapes and compositionally-different reaction rims against the dominant groundmass.
igneous: (of rock) having solidified from lava or magma plagioclase: a sodium- and calcium-rich aluminosilicate mineral plutonic: slowly cooled underground; as opposed to volcanic, which indicates surface eruption brecciated: consisting of broken angular fragments within a fine-grained matrix gabbroic: pertaining to a coarse-grained igneous rock composed mainly of pyroxene and plagioclase pyroxene: a class of silicate minerals containing magnesium, iron and varying amounts of calcium xenocryst: a crystal in an igneous rock which is not derived from the original magma
The non-terrestrial nature of this material can be established from several lines of evidence: 1) the presence in the gabbroic groundmass of rare grains of iron metal (nickel-poor), 2) whole rock FeO/MnO ratios of ~24 (much lower than for terrestrial rocks or even typical eucrites), and 3) oxygen isotope compositions which plot significantly below the established trend for terrestrial rocks (but at higher values than the field for typical eucrites). Although the oxygen isotopic composition is close to values for four anomalous eucrites (Bunburra Rockhole, Emmaville, Asuka 881394 and EET 92023), Erg Chech 002 differs in having highly sodic rather than highly calcic plagioclase.
Figure 2. Back-scattered electron image of the same region as in Figure 1. Darker tones represent lower mean atomic number and brighter tones represent higher mean atomic number.
Figure 3. Magnesium X-ray map of the same region as in Figures 1 and 2. Mg contents range from lowest (black) to medium (blue / green) to highest (red / magenta).
Based on the available data (but with much more to be revealed from further studies) we believe that this achondrite may be derived ultimately from a previously unsampled differentiated, planet-like parent body, possibly one that suffered collisional destruction and dispersion of its crustal rocks early in solar system history. The specimens now under study may have been ejected more recently from remnants of that catastrophic event, which were fortuitously captured into orbit within the main asteroid belt between Mars and Jupiter.
Dr. Tony Irving is a geochemist and meteoriticist at the University of Washington.
Figure 1 by Neil H Buckland.
Figures 2 and 3 by Paul Carpenter at Washington University in St. Louis.