Mount Egerton

Aubrite-anomalous
(Metal-rich, light-REE-enriched)

Found 1941
24° 53′ S., 117° 38′ E. The first fragments of this unusually metal-rich, unbrecciated enstatite achondriteUsed in past as synonym for Aubrites. Present definition from the Meteoritical Bulletin states that this rare class is an "enstatite-rich  achondrite  that has not yet been classified into a group". were found by an Australian prospector, with subsequent recoveries bringing the total combined weight to ~22 kg. Mount Egerton is a very weathered meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and containing minor amounts of Si-bearing metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn both as a host phase and as inclusions within a predominant brown-stained enstatiteA mineral that is composed of Mg-rich pyroxene, MgSiO3. It is the magnesium endmember of the pyroxene silicate mineral series - enstatite (MgSiO3) to ferrosilite (FeSiO3). phase. The meteorite is composed primarily of cm-sized enstatite crystals with inhomogeneously distributed FeNi-metal composing ~21 wt%—a large abundance for any aubriteAubrites are named for the Aubres meteorite that fell in 1836 near Nyons, France. They are an evolved achondrite that is Ca-poor and composed mainly of enstatite (En100) and diopside (En50Wo50) with minor amounts of olivine (Fa0) and traces of plagioclase (An2-8).  They contain large white crystals of enstatite as. Minor abundances of diopside and various sulfides are present (Cr-bearing troiliteBrass colored non-magnetic mineral of iron sulfide, FeS, found in a variety of meteorites., brezinaite, alabanditeMagnesium sulfide found in aubrites and EL chondrites. Its formula is MnS.); notably, Mount Egerton contains no feldsparAn alumino-silicate mineral containing a solid solution of calcium, sodium and potassium. Over half the Earth’s crust is composed of feldspars and due to their abundance, feldspars are used in the classification of igneous rocks. A more complete explanation can be found on the feldspar group page. (Watters and Prinz, 1980).

The metal in Mount Egerton exhibits a very fine pseudo-octahedrite pattern upon etching due to the presence of the Fe–Ni–P-silicide perryite. Perryite only occurs in highly reducedOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants meteorites in which pure magnesian silicates are incorporated in a low-Ni metal host. Small realms of schreibersiteNi-Fe phosphide mineral, (Fe,Ni)3P, yellowish in color and predominantly found in iron and stony-iron meteorites. Schreibersite can also be found in a variety of other meteorites including some acapulcoites, aubrites, enstatite chondrites and achondrites, lunars, ureilites, winonaites and a smattering of other meteorite types like CM, CO and CB. Schreibersite are present in the metal phase. In contrast to the metal in the main-group aubritesAubrites are named for the Aubres meteorite that fell in 1836 near Nyons, France. They are an evolved achondrite that is Ca-poor and composed mainly of enstatite (En100) and diopside (En50Wo50) with minor amounts of olivine (Fa0) and traces of plagioclase (An2-8).  They contain large white crystals of enstatite as, the metal in Mount Egerton exhibits a mostly chondritic highly siderophile elementLiterally, "iron-loving" element that tends to be concentrated in Fe-Ni metal rather than in silicate; these are Fe, Co, Ni, Mo, Re, Au, and PGE. These elements are relatively common in undifferentiated meteorites, and, in differentiated asteroids and planets, are found in the metal-rich cores and, consequently, extremely rare on (HSE) pattern similar to that of enstatite chondritesChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such, particularly that of EL chondriteChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such metal (van Acken et al., 2010; Humayun, 2010; van Acken et al., 2012). Although it has been suggested that Mt. Egerton might represent a sampling of the core–mantleMain silicate-rich zone within a planet between the crust and metallic core. The mantle accounts for 82% of Earth's volume and is composed of silicate minerals rich in Mg. The temperature of the mantle can be as high as 3,700 °C. Heat generated in the core causes convection currents in boundary of the aubrite parent bodyThe body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early in the formation of our Solar System, while others like the asteroid 4-Vesta and Mars are still observable today. (Watters and Prinz, 1980), the unfractionated composition of the metal nodules are inconsistent with an origin from a differentiated coreIn the context of planetary formation, the core is the central region of a large differentiated asteroid, planet or moon and made up of denser materials than the surrounding mantle and crust. For example, the cores of the Earth, the terrestrial planets and differentiated asteroids are rich in metallic iron-nickel. (Barrat et al., 2016). Conversely, the siderophile elementSubstance composed of atoms, each of which has the same atomic number (Z) and chemical properties. The chemical properties of an element are determined by the arrangement of the electrons in the various shells (specified by their quantum number) that surround the nucleus. In a neutral atom, the number of pattern (enriched in compatible siderophiles) of Mount Egerton metal suggests that it could represent a residue formed from crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. of a high degree partial melt (van Acken et al., 2010; Humayun, 2010).

The metal in Mount Egerton and the anomalous iron meteoriteIron meteorites consist mostly of metallic iron alloyed with typically between ~5 to ~30 wt% nickel. The main metal phases are kamacite α-(Fe, Ni) and taenite y-(Fe, Ni). Based on their group classification, they may also contain a small weight percentage of one or more of the following minerals: • Horse Creek (as well as the anomalous irons LEW 85369, LEW 88055, and LEW 88631) has been described as being compositionally similar (i.e., having complementary HSE patterns in metal) to metal in the anomalous enstatite achondriteAn achondrite is a type of stony meteorite whose precursor was of chondritic origin and experienced metamorphic and igneous processes. They have a planetary or differentiated asteroidal origin where the chondritic parent body reached a sufficient size that through heating due to radioactive decay of Al (aluminum isotope) and gravitational NWA 2526, a partial melt residue (after ~20% partial melt extraction) containing ~10–15% metal (Keil and Bischoff, 2008; Humayun et al., 2009; M. Humayun, 2010). Along with the similar partial melt residue Itqiy, these meteorites might share a common origin on an E chondrite-like parent body unique from the Shallowater, EH, EL, and main-group aubrite parent bodies (Keil and Bischoff, 2008). Horse Creek is composed of Si-bearing (2.5 wt%) metal with a perryite component; this is similar to the Si content in host metal in Mount Egerton of 2.03 wt% which also contains perryite. Mount Egerton and Horse Creek also have similar W-isotopic compositions which is unlike that of other aubrites (Barrat et al., 2016). The close similarities shown by Mount Egerton and Horse Creek (and some other aubrites) in trace element abundances, compatible siderophile element enrichments, and Si content of metal, have led some to conclude that Horse Creek is genetically related to aubrites.

Precise trace element and isotopic analyses of a broad sampling of aubrites was conducted by Barrat et al. (2016). They ascertained that all aubrites in the study have identical O-isotopic compositions (considering a degree of contamination in Cumberland Falls and Larned) with a high level of Δ17O homogeneity, consistent with an early stage of extensive melting on the aubrite parent body(ies), possibly involving a magma oceanCompletely molten surfaces of terrestrial planets or moons that formed soon after accretion. Samples returned by the Apollo missions provide evidence of a lunar magma ocean, crystallization of which produced a stratified Moon with a low-density crust formed by accumulation of the mineral plagioclase overlying a higher density mantle of. However, both Mount Egerton and Larned exhibit significant light-REE enrichments compared to the other aubrites, which have light-REE depletions. In addition, the negative Eu anomalies in pyroxenes from Mount Egerton and Larned are less extreme than in the other aubrites. A further difference between main-group aubrites and the Mount Egerton and Larned aubrites can be seen in the siderophile element contents of metal; as described above, these two meteorites contain metal that is much less fractionated than that in other aubrites. In consideration of these differences, Barrat et al. (2016) concluded that Mount Egerton, Horse Creek, Larned, and possibly NWA 2526, likely represent a separate parent body from that of the main-group of aubrites.

In their study of iron meteorite exposure histories, Welten et al. (2008) found that Horse Creek exhibits a complex exposure history consisting of two stages. During the first stage of irradiation, which involved high shielding at a depth of ~60 cm in an object >2 m in diameter, cosmogenic noble gasElement occurring in the right-most column of the periodic table; also called "inert" gases. In these gases, the outer electron shell is completely filled, making them very unreactive. data indicate a CRE age of 100 (+40/–30) m.y., placing it at the high end of the aubrite range. A second stage irradiation lasting ~1 m.y. occurred at a depth of 5–10 cm on a 30–50 cm diameter body. The investigators argue that the cosmogenic radionuclideRadioactive isotope - Atomic nuclide that decays radioactively . and noble gas data for Horse Creek are consistent with that of debris ejected by a minor impact on a km-sized near-Earth object (NEO), which was followed by its rapid delivery to Earth. A possible source object for the Horse Creek meteorite is the Earth-crossing asteroid 3103 Eger, itself possibly derived from a larger main beltBelt located between 2.12 and 3.3 AU from the Sun and located between the orbits of Mars and Jupiter containing the vast majority of asteroids. The asteroid belt is also termed the main asteroid belt or main belt to distinguish it from other asteroid populations in the Solar System such object.

Current spectral studies link the aubrites to a few near-Earth Apollo asteroids, specifically 3103 Eger and 434 Hungaria (Kelley and Gaffey, 2002). These two high-albedo, iron-free asteroids are composed of an enstatite-like silicateThe most abundant group of minerals in Earth's crust, the structure of silicates are dominated by the silica tetrahedron, SiO44-, with metal ions occurring between tetrahedra). The mesodesmic bonds of the silicon tetrahedron allow extensive polymerization and silicates are classified according to the amount of linking that occurs between the, and are of the appropriate size to make them primary candidates for the aubrite source body. Further evidence has been compiled that is consistent with 3103 Eger being the aubrite source body. For example, the time of day in which aubrites have fallen constrains the orbitThe elliptical path of one body around another, typically the path of a small body around a much larger body. However, depending on the mass distribution of the objects, they may rotate around an empty spot in space • The Moon orbits around the Earth. • The Earth orbits around to one similar to that of Eger. In addition, the long cosmic-ray exposure ageTime interval that a meteoroid was an independent body in space. In other words, the time between when a meteoroid was broken off its parent body and its arrival on Earth as a meteorite - also known simply as the "exposure age." It can be estimated from the observed effects of aubrites is consistent with a stable residence on a near-Earth asteroidAsteroids with orbits that bring them within 1.3 AU (195 million km) of the Sun. NEAs are a dynamically young population whose orbits evolve on 100-million-year time scales because of collisions and gravitational interactions with the Sun and the terrestrial planets. These asteroids are probably ejected from the main belt that has a long-lived orbit, similar to that of Eger. Moreover, the orbital parameters derived for Norton County match those of Eger better than all other orbits. Asteroid 3103 Eger was probably once a member of the Hungaria family of asteroids located in the innermost asteroid beltBelt located between 2.12 and 3.3 AU from the Sun and located between the orbits of Mars and Jupiter containing the vast majority of asteroids. The asteroid belt is also termed the main asteroid belt or main belt to distinguish it from other asteroid populations in the Solar System such at 1.9 AUThe astronomical unit for length is described as the "mean" distance (average of aphelion and perihelion distances) between the Earth and the Sun. Though most references state the value for 1 AU to be approximately 150 million kilometers, the currently accepted precise value for the AU is 149,597,870.66 km. The. It was subsequently ejected into an Earth-crossing orbit. Notably, the asteroid 2867 Steins has recently been studied by the Rosetta spacecraft. It was found to have an albedoRatio of the amount of light reflected by an object and the amount of incident light. Albedo is used as a measure of the reflectivity or intrinsic brightness of an object. A white, perfectly reflecting surface has an albedo of 1.0 while a black perfectly absorbing surface would have an and spectral properties consistent with those of an aubrite in having an abundance of CaS or oldhamiteMn-Ca sulfide, (Mn,Ca)S, is a pale to dark brown accessory mineral found in minor amounts in highly reduced meteorites such as many enstatite chondrites, and some aubrites and enstatite achondrites. Oldhamite in enstatite chondrites likely formed by solar nebular gas condensation. CaS Oldhamite was also found in the most fresh (Abell et al., 2008).

Mount Egerton has a cosmic-ray exposure age of 28 (±4) m.y., which is among the lowest of all the aubrites. Cosmogenic production rates indicate that it also had a small pre-atmospheric diameter of ~60 cm. Continued searches of the area have resulted in the recovery of thousands of additional small fragments of this aubrite for researchers and collectors alike. The specimens shown above are a cm-sized 2.7 g metallic fragment similar in composition to the iron meteorite Horse Creek, and a 1.2 g fragment of enstatite silicate devoid of all metal.