Enstatite Chondrite
Type of 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 high in 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 Click on Term to Read More 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 and also referred to as E-chondrites. Although they contain substantial amounts of Fe, it is in the form of Ni-Fe 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 or sulfide rather than as oxides in silicates. Their 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 Click on Term to Read More nature indicates that they formed in an area of the solar nebulaThe primitive gas and dust cloud around the Sun from which planetary materials formed. that was very poor in oxygenElement that makes up 20.95 vol. % of the Earth's atmosphere at ground level, 89 wt. % of seawater and 46.6 wt. % (94 vol. %) of Earth's crust. It appears to be the third most abundant element in the universe (after H and He), but has an abundance only Click on Term to Read More (low oxygen fugacityUsed to express the idealized partial pressure of a gas, in this case oxygen, in a nonideal mixture. Oxygen fugacity (ƒO2) is a measure of the partial pressure of gaseous oxygen that is available to react in a particular environment (e.g. protoplanetary disk, Earth's magma, an asteroid's regolith, etc.) and Click on Term to Read More). The oxygen isotopeOne of two or more atoms with the same atomic number (Z), but different mass (A). For example, hydrogen has three isotopes: 1H, 2H (deuterium), and 3H (tritium). Different isotopes of a given element have different numbers of neutrons in the nucleus. Click on Term to Read More composition plots close to 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. Click on Term to Read More line (TFL). Previous research suggests that E-chondrites formed in the inner regions of the protoplanetary diskFlattened and rotating disk of dense gas and dust/solids orbiting a young star from which planets can eventually form. Click on Term to Read More, possibly inside 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 of Mercury, and they are therefore considered to be candidates for the building blocks of the terrestrial planetsRocky planets: Mercury, Venus, Earth, and Mars. These planets have physical characteristics, chemical composition and internal structure similar to the Earth. The terrestrial planets have 0.4% of the total mass of all the planets in the Solar System. Some large satellites of planets are also similar to the characteristics of.
Important note: Many websites incorrectly identify the difference between EH and EL 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 as being based on amount of bulk iron: EH chondrites contain ~30% Fe (high bulk iron), whereas EL chondrites have ~25% Fe (low bulk iron). This determination was based on a low number of E-chondrites available at the time and no longer correct.
The reduced character of E-chondrites is reflected in their mineralogy. They consist of nearly pure end member enstatite, 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, Ti-Cr-bearing troilite(FeS), and Mg-, Ca-, Mn-bearing sulfides. The latter sulfides are rare minerals and include niningerite (MgS), alabandite(MnS), and 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 (CaS), mostly unobserved in any terrestrial rocks. Unlike other 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 classes, troiliteBrass colored non-magnetic mineral of iron sulfide, FeS, found in a variety of meteorites. Click on Term to Read More (typically 5–17 wt%) and Si-bearing Fe,Ni metal (17–28 wt% ) are major constituents. Due to their bulk composition, enstatite chondrites have traditionally been subdivided into EH (high bulk iron) and EL (low bulk iron) chondrites.
Updated Enstatite Chondrite Group Characteristics
| EH | EL | |||
|---|---|---|---|---|
| ChondruleRoughly 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 Size | 0.2 mm | 0.6 mm | ||
| Metal Abundance | ~10 vol% | ~10 vol% | ||
| Si Content in Metal | ~3 wt% | ~1 wt% | ||
| Mineral Assemblages in Metal | niningerite (MgS) & perryite (Fe-Ni silicide) | ferroan alabanditeMagnesium sulfide found in aubrites and EL chondrites. Its formula is MnS. Click on Term to Read More ((Fe,Mn)S) | ||
However, recent research based on the systematic evaluation of 80 different enstatite chondrites has revealed new subgroups.
New enstatite chondrite subgroups:
| EHa | EHb | ELa | ELb | |
|---|---|---|---|---|
| Troilite | Cr < 2 wt% | Cr > 2 wt% | Cr < 2 wt% | Cr > 2 wt% |
| (Mn,Mg,Fe)S | Fe < 20 wt% | Fe > 20 wt% | Fe < 20 wt% | Fe > 20 wt% |
| Daubreelite | Abundant | Missing | Abundant | Missing |
| Kamacite | Ni < 6.5 wt% | Ni > 6.5 wt% | Ni < 6.5 wt% | Ni > 6.5 wt% |
Since mineral chemistry does not correlate with the petrologic typeMeasure of the degree of aqueous alteration (Types 1 and 2) and thermal metamorphism (Types 3-6) experienced by a chondritic meteorite. Type 3 chondrites are further subdivided into 3.0 through 3.9 subtypes., the degree of recrystallization is the only fundamental feature for defining the petrologic types of E chondrites. As it is not possible to distinguish type 4 from type 3 by the characteristics of opaque phases, it is suggested that 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 Click on Term to Read More can only be a mineral of the type 3 E chondrite subgroups.
Significant portions of the above definition were derived from the 2018 paper, “Chemical variations of sulfides and metal in enstatite chondrites—Introduction of anew classification scheme.” M. Weyrauch, M. Horstmann, and A. Bischoff.
Another paper with interesting information is the 2009 paper, “ENSTATITE CHONDRITE PHYSICAL PROPERTIES: DENSITYMass of an object divided by its volume. Density is a characteristic property of a substance (rock vs. ice, e.g.). Some substances (like gases) are easily compressible and have different densities depending on how much pressure is exerted upon them. The Sun is composed of compressible gases and is much Click on Term to Read More, POROSITYThe volume percentage of a rock that consists of void space. Vesicular porosity is a type of porosity resulting from the presence of vesicles, or gas bubbles, in igneous rock such as the pumice presented here. Vesicular porosity is very rare in meteorites and is often associated with slag, one Click on Term to Read More AND MAGNETIC
SUSCEPTIBILITY.” Robert J. Macke SJ, et. al.
