Indarch

EH4
(EHa4 in Weyrauch et al., 2018)

Fell April 7, 1891
39° 45′ N., 46° 40′ E. At 8:10 P.M., people in Azerbaydzhan, SSR, USSR, witnessed a fireballA fireball is another term for a very bright meteor, generally brighter than magnitude -4, which is about the same magnitude of the planet Venus as seen in the morning or evening sky. A bolide is a special type of fireball which explodes in a bright terminal flash at its end, often with visible fragmentation. accompanied by detonations. The next morning, a single 27 kg stone was recovered. Indarch has experienced minimal impact alteration, exhibiting weak shock metamorphismMetamorphism produced by hypervelocity impact between objects of substantial size moving at cosmic velocity (at least several kilometers per second). Kinetic energy is converted into seismic and heat energy almost instantaneously, yielding pressures and temperatures far in excess those in normal terrestrial metamorphism. On planetary bodies with no atmosphere, smaller (S3) at shock pressures of 5–10 GPa. This produced planar fractures in olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion and twinned clinoenstatite, along with metallic shock veins.

The Van Schmus–Wood (1967) scheme for petrographic type has been modified for 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). 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, establishing both a textural type (3–7), reflecting peak metamorphicRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. temperature, and a mineralogical type (α–δ), pertaining to the cooling history (Zhang and Sears, 1996; Quirico et al., 2011). Under this classification scheme, Indarch has thermometers that give it a classification of EH4β,γ.

Weyrauch et al. (2018) analyzed the 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 and chemical data from 80 enstatite chondrites representing both EH and EL groups and spanning the full range of petrologic types for each group. They found that a bimodality exists in each of these groups with respect to both the Cr content in troiliteBrass colored non-magnetic mineral of iron sulfide, FeS, found in a variety of meteorites. and the Fe concentration in niningerite and alabanditeMagnesium sulfide found in aubrites and EL chondrites. Its formula is MnS. (endmembers of the [Mn,Mg,Fe] solid solutionCompositional variation resulting from the substitution of one ion or ionic compound for another ion or ionic compound in an isostructural material. This results in a mineral structure with specific atomic sites occupied by two or more ions or ionic groups in variable proportions. Solid solutions can be complete (with series present in EH and EL groups, respectively). In addition, both the presence or absence of daubréelite and the content of Ni in 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 were demonstrated to be consistent factors for the resolution of four distinct E 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 groups: EHa, EHb, ELa, and ELb (see table below).

A few other E chondrites with intermediate mineralogy have also been identified, including LAP 031220 (EH4), QUE 94204 (EH7), Y-793225 (E-an), LEW 87223 (E-an), and PCA 91020 (possibly related to LEW 87223). Studies have determined that these meteorites were not derived from the EH or EL source through any metamorphic processes, and some or all of them could represent separate E chondrite asteroids. The revised E chondrite classification scheme of Weyrauch et al. (2018) including selected examples from their 80-sample study can be found here. It was determined that Indarch is a member of the EHa subgroup.

Indarch contains both silicate-rich and metal-rich 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 embedded within a sulfide-rich matrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents.. A two-stage cooling history has been suggested to explain the reversed zoning in the chondrules. Other minerals present in Indarch include sub-µm-sized, presolar SiC grains in a concentration of 1.3 ppmParts per million (10). (Huss, 1990). These have a grain size closely matching that of unprocessed circumstellar grains, and S-isotopic compositions consistent with an origin on AGB starsStars on the Asymptotic Giant Branch, which represents a late stage of stellar evolution that all stars with initial masses < 8 Msun go through. At this late stage of stellar evolution, gas and dust are lifted off the stellar surface by massive winds that transfer material to the interstellar (Gyngard et al., 2012). The silicon-containing nitride, nierite, also occurs, which some investigators concluded was formed by exsolutionSegregation, during cooling, of a homogeneous solid solution into two or more different solids. of kamacite, perryite, and 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 during 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. metamorphism (Alexander et al., 1994). However, an advanced isotopic study conducted by Leitner et al. (2018) led to their contention that the majority of the silicon nitride was formed by nebular condensation/precipitation processes (a rare component is presolar) prior to its incorporation into the EC parent body. Micro- and nano-scale diamonds are present in Indarch at a concentration of ~17 ppm, and the sulfide minerals 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 and niningerite have also been identified.

By using 53Mn/53Cr ratios as a chronometer for absolute ages, Shukolyukov and Lugmair (2004) estimated the age of Indarch to be 4.565 b.y. A similar age of ~4.563 b.y. was determined by Busfield et al. (2008) based on I–Xe systematics, while Moseley et al. (2011) calculated an age based on Mn–Cr systematics and anchored to D’Orbigny of ~4.5674 b.y. These ages are similar to that of EH4 Abee, but slightly older than EL6 Khairpur, possibly reflecting Khairpur’s extended cooling history. The K–Ar closure age as determined by Bogard et al. (2010) occurred ≥4.35 b.y., and evidence indicates a later impact-degassing event 4.25 b.y. ago. A Rb–Sr isochron gives an age of 4.52 (±0.15) b.y., while a corrected Rb–Sr age gives 4.50 b.y. Indarch has a matrix CRE age based on 3He, 21Ne, and 38Ar of 12.1 (±2.5) m.y. (Eugster et al., 2007).

Although E chondrites and 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 share a common O-isotopic signature, some chemical and mineralogical differences exist which had previously cast doubt on their formation on a common parent body. Some of these differences include the higher abundance of Ti and forsteritePure* magnesium end-member (Mg2SiO4) of the olivine solid solution series and an important mineral in meteorites. When magnesium (Mg) is completely substituted by iron, it yields the the pure Fe-olivine end member, fayalite (Fe2SiO4). The various Fe and Mg substitutions between these two end-members are described based on their forsteritic (Fo) in aubritic sulfides than in E chondrites. A scenario reconciling these differences has been presented in light of an experiment in which an E chondrite was systematically melted in a highly 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, oxygen-depleted environment. In the experiment, as the silicate-melt reached a temperature range of 1000–1300°C having a degree of partial meltingAn igneous process whereby rocks melt and the resulting magma is comprised of the remaining partially melted rock (sometimes called restite) and a liquid whose composition differs from the original rock. Partial melting occurs because nearly all rocks are made up of different minerals, each of which has a different melting of 20%, the metal–sulfide component began to migrate out of the 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. At 1450°C, a completely separated 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 component could have begun to establish a metallic 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. on its parent body. Since the sulfides melted at temperatures as low as 1000°C, it is demonstrated that aubritic sulfides cannot be a product of nebular synthesis as previously speculated. Instead, the tranfer of S and Ca from the S-rich silicate melts resulted in magmatic crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. of oldhamite (CaS). Additionally, a phase was reached at 1500°C in which tectosilicate was 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 to Si within the metallic melt, with the subsequent crystallization of forsterite. Moreover, Ti-rich troilite crystallized from a combination of an Fe-rich sulfide melt, and a mixed-sulfide melt. All of the results of the experiment are consistent with a derivation of the aubrites from an E chondrite-type precursor in a strongly reducing, oxygen-depleted environment. Previous studies employing multiple lines of evidence including chemical, petrographic, metamorphic, and 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 data, suggest that the EL and EH chondrites originated in different layers of the same asteroidal parent body. More recently, very precise isotopic measurements were made of a statistically larger sampling of E chondrites and aubrites. Although their O-isotopic data were indeed identical, a three-isotope plot distinguished the EH group from the EL and 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 groups by its slightly steeper slope; the plots of the EL and aubrite groups were co-linear with the terrestrial fractionationConcentration or separation of one mineral, element, or isotope from an initially homogeneous system. Fractionation can occur as a mass-dependent or mass-independent process. line. A third grouplet with intermediate mineralogy has recently been identified, represented by the 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 Y-793225. Studies have determined that it was not derived from material associated with the EH or the EL groups through any metamorphic processes, and therefore could represent a unique enstatite parent body. The Shallowater meteorite is also widely considered to originate from a unique enstatite parent object.

The iron-rich, oxygen-poor composition of Indarch, as well as its greater depletion of refractories than is found on the Earth, has led to speculation that E chondrites might have once been a part of the pre-differentiated outer layer of Mercury. However, reflectance spectrometry has determined that E-type and M-type asteroids are similar to E chondrites, and that they occupy stable orbits between 1.8 and 3.2 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. These findings suggest that the 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 is where they originated, or more likely, to where they were collisionally and/or gravitationally relocated. A heliocentricCentered around a sun. Our own Solar System is centered around the Sun so that all planets such as Earth orbit around the Sun. Note that 25% of Americans incorrectly believe the Sun revolves around the Earth. distance of ~2.0–2.9 AU was calculated for two E chondrites on the basis of their implanted solar 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. concentrations (Nakashima et al., 2004). By utilizing Mn–Cr isotopic systematics, Shukolyukov and Lugmair (2004) concluded that the E chondrites formed at a location closer to the Sun—between at least 1 AU outward to 1.4 AU—than that which they now occupy. Furthermore, an anomalous light N component found proportionately in carbonaceous and E chondrites but not on Earth, and which is almost certainly of nucleosynthetic origin, attests to a similar heliocentric location for the formation of these bodies.

Details of a computer-based model (Blander et al., 2009) of the formation history of E chondrites can be found on the Sahara 97096 page. The specimen of Indarch shown above is a 1.0 g cut fragment.