| Enstatite chondrite | |
|---|---|
| — Class — | |
 Section of the Abee meteorite, an enstatite chondrite, on display at theRoyal Ontario Museum | |
| Compositional type | Stony |
| Type | Chondrite |
| Parent body | 16 Psyche |
| Total known specimens | ~200 |
| Alternative names | E-type chondrites |
Enstatite chondrites (E-type chondrites) are a rare form ofmeteorite, rich in the mineralenstatite. Only about 200 E-Type chondrites are currently[when?] known,[1] comprising about 2% of thechondrites that fall on Earth.[1] There are two main subtypes: EH and EL, classified based on their iron content.
E-type chondrites are among the most chemicallyreduced rocks known, with most of theiriron taking the form of metal or sulfide rather than an oxide. They tend to be high in the mineralenstatite (MgSiO3), from which they derive their name.[1] Based on spectral analysis, it has been suggested that the asteroid16 Psyche may be the common parent for this type of meteorite.
Unlike most other chondrites, the minerals in enstatite chondrites contain almost no iron oxide; they are the most oxygen-poor silicate rocks known. They were supposed to be the driest objects in theSolar System but a recent study shows they contain sufficient hydrogen to have delivered to Earth at least three times the mass of water in its oceans.[2] Metallic Fe-Ni (iron-nickel) and Fe-bearing sulfide minerals contain nearly all of the iron in this type of meteorite. Enstatite chondrites contain a variety of unusual minerals that can only form in extremely reducing conditions, includingoldhamite (CaS),niningerite (MgS),perryite (Fe-Ni silicide), and alkali sulfides (e.g.,djerfisherite andcaswellsilverite). All enstatite chondrites are dominantly composed of enstatite-rich chondrules plus abundant grains of metal and sulfide minerals. Dusty matrix material is uncommon and refractory inclusions are very rare. Chemically, enstatite chondrites are very low in refractory lithophile elements. Their oxygen isotopic compositions are intermediate between ordinary and carbonaceous chondrites, and are similar to rocks found on the Earth and Moon. Their lack of oxygen content may mean that they were originally formed near the center of the solar nebula that created theSolar System, possibly within the orbit ofMercury. Most enstatite chondrites have experienced thermal metamorphism on the parent asteroids. They are divided into two groups:[3][4]
The largest known recorded E-type chondrite fall happened in the town ofAbee, inAlberta in 1952. The 107 kilograms (236 lb) stone fell in a farmer's wheat field, creating an impact pit 0.7 m in diameter and 1.5 m deep.[5] Based on estimates of its velocity and inclination, it is thought to have arrived at a relatively low-speed, low-inclination orbit that had aperihelion near 0.95 AU and anaphelion probably close to 2.74 AU.[6] It is classified as being a shock-meltedbreccia, its minerals having recrystallized after the impact that knocked it off of its parent body.[7] The meteorite was acquired by theGeological Survey of Canada,[8] and a large sample of it is on display at theRoyal Ontario Museum.
The largest known E-type chondrite in theasteroid belt may be21 Lutetia, with a diameter of approximately 100 kilometres (62 mi),[9] based on observations from theRosetta spacecraft, ESO'sNew Technology Telescope,NASA Infrared Telescope Facility, and theSpitzer Space Telescope.
