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 Click on Term to Read More (main-group)
click on photo for a magnefied view Found 2010
no coordinates recorded A small, relatively fresh stone weighing 50.5 g was found in the Sahara and designated NWA 6350. Analysis was conducted by the University of Washington in Seattle (A. Irving), and NWA 6350 was initially determined to be a likely pairing with the fusionProcess in which two lighter atomic nuclei combine to form a heavier atomic nucleus. Very high temperatures are normally required in order for atomic nuclei to collide with sufficient energy to overcome the Coulomb barrier (their mutual electrostatic repulsions). Fusion that occurs under high-temperature conditions is called thermonuclear fusion. Fusion Click on Term to Read More crusted, 39.1 g aubrite, NWA 5217, found in 2007 in Morocco. Thereafter, it was considered likely that it was a member of a large pairing group, additionally comprising the NWA-series numbers 4537, 4799, 4832, 4871, 5217, 5419, 6193, 6675, and probably the largest and least weathered mass, 7214, all of which together weigh 5,047.6 g.
Northwest Africa 6350 is a rare unbrecciated (common only to the 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 Click on Term to Read More
Mt. Egerton and Shallowater), cumulate-textured aubrite, formed through igneous processes and fractional crystallation. Northwest Africa 6350 consists of a fine- to medium-grained aggregate of mostly pure 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). Click on Term to Read More
along with minor amounts of sodic plagioclaseAlso referred to as the plagioclase feldspar series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous solid solution of calcium and sodium: (Na1-x,Cax)(Alx+1,Si1-x)Si2O8 where x = 0 to 1. The Ca-rich end-member is called anorthite (pure anorthite has formula: CaAl2Si2O8) and the Na-rich end-member is albite Click on Term to Read More
, daubreelite, Si-bearing kamaciteMore common than taenite, both taenite and kamacite are Ni-Fe alloys found in iron meteorites. Kamacite, α-(Fe,Ni), contains 4-7.5 wt% Ni, and forms large body-centered cubic crystals that appear like broad bands or beam-like structures on the etched surface of a meteorite; its name is derived from the Greek word Click on Term to Read More
, Cr-bearing troiliteBrass colored non-magnetic Fe sulfide, FeS, found in a variety of meteorites.
, 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 Click on Term to Read More
, alabanditeMagnesium sulfide found in aubrites and EL chondrites. Its formula is MnS. Click on Term to Read More
, niningerite, caswellsilverite, graphiteOpaque form of carbon (C) found in some iron and ordinary chondrites and in ureilite meteorites. Each C atom is bonded to three others in a plane composed of fused hexagonal rings, just like those in aromatic hydrocarbons. The two known forms of graphite, α (hexagonal) and β (rhombohedral), have Click on Term to Read More
, and rare zincian brezinaite (Bunch and Wittke, NAU). The enstatite grains exhibit a preferred orientation.
According to the authoritative source, the Meteoritical Bulletin Database, out of a total of nearly 6,000 meteorites recovered thus far from the desert regions of Northwest Africa, only a small percentage are aubrites. Besides NWA 6350 and its large pairing group, the anomalous aubrite NWA 1235
was determined to be a unique 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 Click on Term to Read More 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 26Al (aluminum isotope) and gravitational Click on Term to Read More
genetically related to the enstatite 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 Click on Term to Read More
clan. Although NWA 2736 was initially classified as an aubrite, in-depth studies conducted by Bunch et al. (2006)
examining numerous paired samples (with various NWA-series designations) revealed the presence of chondrulesRoughly spherical aggregate of coarse crystals formed from the rapid cooling and solidification of a melt at ~1400 ° C. Large numbers of chondrules are found in all chondrites except for the CI group of carbonaceous chondrites. Chondrules are typically 0.5-2 mm in diameter and are usually composed of olivine Click on Term to Read More
. Therefore, that pairing group has been reclassified as an EL3 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 Click on Term to Read More
In addition to these classified aubrites, three separate NWA stones have been classified as enstatite achondrites: 1) the 42.9 g NWA 2526 found in 2003 contains 10% 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 Click on Term to Read More
; 2) the 132.8 g NWA 1840 found in 2003 has many features similar to Shallowater and might be related to that unique enstatite 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. Click on Term to Read More
, or it could represent a 5th enstatite parent body; 3) the 483 g NWA 4642 was found in 2007.
A large proportion (~40%) of aubrites are witnessed falls, which is thought to reflect the fact that these highly reduced meteorites are particularly susceptible to terrestrial weathering once they arrive on Earth. Although NWA 6350 is a comparatively fresh meteorite that has preserved its accessory minerals, the original FeNi-metal component in the form of kamacite has been converted to secondary weathering products manifest as limonite veinlets and orange staining along enstatite grain boundaries. Since all of the stones constituting the pairing group were recovered throughout the years 2005–2010, they have experienced a range of terrestrial weathering processes. They now exhibit weathering grades of W0/1–W5, yet mineralInorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous Click on Term to Read More
phases associated with the aubrite group are still prevalent in them all (Irving and Kuehner, UWS). The surprisingly rapid alteration processes that affect all aubrites in Earth’s oxidizingOxidation 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 Click on Term to Read More
and wet environs is demonstrated visually in the following photos of stones from a common fallMeteorite seen to fall. Such meteorites are usually collected soon after falling and are not affected by terrestrial weathering (Weathering = 0). Beginning in 2014 (date needs confirmation), the NomComm adopted the use of the terms "probable fall" and "confirmed fall" to provide better insight into the meteorite's history. If Click on Term to Read More
. On the left, shown courtesy of Darryl Pitt, is the fresh (W0/1) 2.2 kg NWA 7214 stone that was recovered in 2006, exhibiting a high content of FeNi-metal flakes throughout with virtually no visible oxidationOxidation 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 Click on Term to Read More
. The two much smaller stones—the 510 g NWA 6675 in the middle and the 50.5 g NWA 6350 stone on the right—had both remained in the terrestrial environment until their recovery in 2010 and have sustained considerable weathering; the 10× larger NWA 6675 stone has experienced significantly less alteration than NWA 6350 probably due to their comparative sizes.
Keil (2010) suggests that the extremely reducingOxidation 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 Click on Term to Read More
conditions under which aubrites formed is evidence for a location within 1 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 Click on Term to Read More
of the SunOur parent star. The structure of Sun's interior is the result of the hydrostatic equilibrium between gravity and the pressure of the gas. The interior consists of three shells: the core, radiative region, and convective region. Image source: http://eclipse99.nasa.gov/pages/SunActiv.html. The core is the hot, dense central region in which the
, but on a parent body other than any of the known E 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 Click on Term to Read More
or the Shallowater source object. The size of the differentiated aubrite planetesimal(s) is constrained by those processes which caused it to melt. Arguments suggesting that the heat source was the decay of short-lived radionuclides such as 26
Al have not been reconciled with the apparent low Al and plagioclase contents in aubrites. In a similar manner, John T. Wasson (2016) presented evidence that the slow heating generated entirely by the decay of 26
Al is insufficient to melt asteroids, and that an additional heat source would have been required; e.g.
, the rapid heating incurred from major impact events. He determined that the canonical 26
Al ratio of 0.000052 is much too low to cause any significant melting, and that a minimum ratio of 0.00001 would be required to produce a 20% melt fraction on a well-insulated body having a significant concentration of 26
Al. For example, the initial ratio of 0.0000004–0.0000005 calculated for the angrites Sah 99555 and D’Orbigny based on their 26
Mg isochrons is too low to have generated any significant melting without an additional heat source. It has been suggested that relatively small planetesimalsHypothetical solid celestial body that accumulated during the last stages of accretion. These bodies, from ~1-100 km in size, formed in the early solar system by accretion of dust (rock) and ice (if present) in the central plane of the solar nebula. Most planetesimals accreted to planets, but many – Click on Term to Read More
such as the aubrite planetesimal(s) might have been just the required size to allow heating by induction in the plasmaFourth state of matter: a gas in which many or most of the atoms are ionized. In the plasma state the atoms have split into positive ions and negative electrons, which can flow freely, so the gas becomes electrically conducting and a current can flow. Click on Term to Read More
environment of the T Tauri Sun.
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 which 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 Click on Term to Read More
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 Click on Term to Read More
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 Click on Term to Read More
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 Click on Term to Read More
at 1.9 AU. It was subsequently ejected into an Earth-crossing orbit. Notably, the asteroid 2867 Steins was recently studied by the Rosetta spacecraft and 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 Click on Term to Read More
and spectral properties consistent with those of an aubrite (with an abundance of CaS or oldhamite) (Abell et al.
, 2008); however, the unique texture and mineralogy of NWA 5217/6350 indicate it derives from a significantly larger parent body.
For additional information on the formation of the aubrite group visit the Mayo Belwa
page. The specimen of NWA 6350 shown above is a 1.03 g partial slice.