Enstatite chondrite
Encyclopedia
Enstatite chondrites are a rare form of meteorite
thought to comprise only about 2% of the chondrites that fall on Earth. Only about 200 E-Type chondrites are currently known.
E-type chondrites are among the most chemically reduced
rocks known, with most of their iron
taking the form of metal or sulfide rather than an oxide. They tend to be high in the mineral enstatite
(MgSiO3), from which they derive their name. Based on spectral analysis, it has been suggested that the asteroid 16 Psyche
may be the common parent for this type of meteorite. Some examples may have originated from the planet Mercury
.
Unlike most other chondrites, the minerals in enstatite chondrites contain almost no iron oxide; they are the most oxygen-poor objects known. 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, including oldhamite (CaS), niningerite
(MgS), perryite (Fe-Ni silicide), and alkali sulfides (e.g., djerfisherite and caswellsilverite). 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 the solar system
, possibly within the orbit of Mercury
. Most enstatite chondrites have experienced thermal metamorphism on the parent asteroids. They are divided into two groups:
The largest known recorded E-type chondrite fall happened in the town of Abee, in Alberta
in 1952. The 107 kg stone fell in a farmer's wheat field, creating an impact pit 0.7 m in diamter and 1.5 m deep. Based on estimates of its velocity and inclination, it is thought to have arrived on a relatively low-speed, low-inclination orbit that had a perihelion near 0.95 AU and an aphelion probably close to 2.74 AU. It in classified as being a shock-melted breccia
, its minerals having recrystalized after the impact that knocked it off of its parent body. The meteorite was acquired by the Geological Survey of Canada, and a large sample of it is on display at the Royal Ontario Museum
.
The largest known E-type chondrite in the asteroid belt
may be 21 Lutetia
, with a diameter of approximately 100 km, based on observations from the Rosetta
spacecraft, ESO's New Technology Telescope
, NASA Infrared Telescope Facility, and the Spitzer Space Telescope
.
Meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
thought to comprise only about 2% of the chondrites that fall on Earth. Only about 200 E-Type chondrites are currently known.
E-type chondrites are among the most chemically reduced
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
rocks known, with most of their iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
taking the form of metal or sulfide rather than an oxide. They tend to be high in the mineral enstatite
Enstatite
Enstatite is the magnesium endmember of the pyroxene silicate mineral series enstatite - ferrosilite . The magnesium rich members of the solid solution series are common rock-forming minerals found in igneous and metamorphic rocks...
(MgSiO3), from which they derive their name. Based on spectral analysis, it has been suggested that the asteroid 16 Psyche
16 Psyche
16 Psyche )is one of the ten most massive main-belt asteroids. It is over 200 kilometers in diameter and contains a little less than 1% of the mass of the entire asteroid belt. It is the most massive of the metallic M-type asteroids....
may be the common parent for this type of meteorite. Some examples may have originated from the planet Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
.
Unlike most other chondrites, the minerals in enstatite chondrites contain almost no iron oxide; they are the most oxygen-poor objects known. 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, including oldhamite (CaS), niningerite
Niningerite
Niningerite is a magnesium-iron-manganese sulfide mineral with the chemical formula S that is found in enstatite chondrite meteorites. Niningerite is the magnesium-dominant analog of keilite. This mineral is named after Harvey H. Nininger....
(MgS), perryite (Fe-Ni silicide), and alkali sulfides (e.g., djerfisherite and caswellsilverite). 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 the solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
, possibly within the orbit of Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
. Most enstatite chondrites have experienced thermal metamorphism on the parent asteroids. They are divided into two groups:
- EH (high enstatite) chondrites contain small chondrules (~0.2 mm) and high ratios of siderophile elements to silicon. Somewhat more than 10% of the rock is composed of metal grains. A diagnostic feature of EH chondrites is that the Fe-Ni metal contains ~3 wt% elemental silicon.
- EL (low enstatite) chondrites contain larger chondrules (>0.5 mm), and low ratios of siderophile elements to silicon. Fe-Ni metal contains ~1 wt% Silicon.
The largest known recorded E-type chondrite fall happened in the town of Abee, in Alberta
Alberta
Alberta is a province of Canada. It had an estimated population of 3.7 million in 2010 making it the most populous of Canada's three prairie provinces...
in 1952. The 107 kg stone fell in a farmer's wheat field, creating an impact pit 0.7 m in diamter and 1.5 m deep. Based on estimates of its velocity and inclination, it is thought to have arrived on a relatively low-speed, low-inclination orbit that had a perihelion near 0.95 AU and an aphelion probably close to 2.74 AU. It in classified as being a shock-melted breccia
Breccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
, its minerals having recrystalized after the impact that knocked it off of its parent body. The meteorite was acquired by the Geological Survey of Canada, and a large sample of it is on display at the Royal Ontario Museum
Royal Ontario Museum
The Royal Ontario Museum is a museum of world culture and natural history in Toronto, Ontario, Canada. With its main entrance facing Bloor Street in Downtown Toronto, the museum is situated north of Queen's Park and east of Philosopher's Walk in the University of Toronto...
.
The largest known E-type chondrite in the asteroid belt
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...
may be 21 Lutetia
21 Lutetia
21 Lutetia is a large main-belt asteroid of an unusual spectral type. It measures about 100 kilometers in diameter . It was discovered in 1852 by Hermann Goldschmidt, and is named after Lutetia, the Latin name for Paris....
, with a diameter of approximately 100 km, based on observations from the Rosetta
Rosetta (spacecraft)
Rosetta is a robotic spacecraft of the European Space Agency on a mission to study the comet 67P/Churyumov–Gerasimenko. Rosetta consists of two main elements: the Rosetta space probe and the Philae lander. The spacecraft was launched on 2 March 2004 on an Ariane 5 rocket and will reach the comet by...
spacecraft, ESO's New Technology Telescope
New Technology Telescope
The New Technology Telescope or NTT is an Alt-Az, 3.58-metre Richey-Chretien telescope part of the European Southern Observatory and began operations in 1989. It is located in Chile at the La Silla Observatory and was an early pioneer on the use of active optics...
, NASA Infrared Telescope Facility, and the Spitzer Space Telescope
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...
.