Enstatite chondrites
Chondrites formed with almost no oxygen to spare, full of rare minerals unknown on Earth's surface.
- Classification
- Stony meteorites › Chondrites › Enstatite chondrites
- Share of witnessed falls
- about 1.6%
- In our catalogue
- 533 meteorites, 17 seen to fall
- Heaviest
- Northwest Africa 4544, 200 kg
- Earliest recorded fall
- Pillistfer, 1863
- Most recent fall
- Neuschwanstein, 2002
Enstatite (E) chondrites formed in extremely reducing conditions, with so little available oxygen that iron is almost all in metal and sulphides rather than in silicates. Their main silicate is enstatite, an almost iron-free pyroxene, and they contain unusual minerals such as oldhamite (calcium sulphide) and niningerite. They make up about 1.6% of falls.
They come in two groups: EH (high iron, more metal, small chondrules) and EL (low iron). Their oxygen isotopes are almost identical to Earth's and the Moon's, which has led some scientists to suggest that material like this made up much of the early Earth.
Mineralogy and chemistry
The main silicate is enstatite, magnesium-rich pyroxene with almost no iron. Iron is present instead as kamacite metal, which contains dissolved silicon, and as sulphides. Elements that normally bond with oxygen, such as calcium, magnesium, manganese, chromium and even titanium, form sulphides here: oldhamite (calcium sulphide), niningerite or alabandite, daubréelite (an iron-chromium sulphide) and osbornite (titanium nitride). Silicon-bearing nitrides such as sinoite and phosphides such as schreibersite are also present.
Oxygen isotopes of enstatite chondrites lie on the terrestrial line, and their isotopes of nitrogen, chromium, titanium, nickel, molybdenum and ruthenium are also the closest of any meteorites to Earth's. That has made them a leading model for the material that built Earth, though their silicon-rich, magnesium-poor chemistry does not match Earth exactly.
Parent bodies and formation
Enstatite chondrites probably formed in the inner Solar System, in a region poor in water and oxygen. EH and EL chondrites have different chemistries and came from at least two separate parent bodies. A study published in 2020 found that they contain enough hydrogen to have supplied Earth with several times the water in its oceans, suggesting that Earth could have obtained much of its water from its own building blocks. Some researchers link them to E-type and some M-type asteroids; the asteroid 21 Lutetia, visited by the Rosetta spacecraft in 2010, has been compared with them. The aubrites are melted rocks from similar, very reduced material.
Abundance and famous examples
About 1.6% of falls are enstatite chondrites, split roughly equally between EH and EL. Abee (EH4, Canada, 1952), Indarch (EH4, Azerbaijan, 1891), Qingzhen (EH3, China, 1976), Hvittis (EL6, Finland, 1901), Pillistfer (EL6, Estonia, 1863) and Eagle (EL6, Nebraska, 1947) are well-known falls.
Shock and impact melts
Many enstatite chondrites have been strongly shocked or melted by impacts, and some rocks once thought to be achondrites are now seen as impact melts of enstatite chondrite. Their minerals act as thermometers: the way niningerite and alabandite take up iron, manganese and magnesium records the temperatures each rock reached, and shows that different stones cooled at very different rates, as expected if their parent bodies were broken apart and reassembled while still warm.
How to recognise one
Fresh enstatite chondrites are dark grey and contain abundant metal, so they are heavy and strongly magnetic. The silicates are pale but often darkened by fine metal and sulphide. Their unusual sulphides break down quickly in moist air: oldhamite reacts with water and can give off a faint smell of rotten eggs, and finds rust rapidly. They need laboratory confirmation.
Groups
EH chondrites
High-iron enstatite chondrites.
170 in the catalogue
EL chondrites
Low-iron enstatite chondrites.
120 in the catalogueFamous examples
- Abee EH4, Canada, 1952 · 107 kg
- Indarch EH4, Azerbaijan, 1891 · 27 kg
- Hvittis EL6, Finland, 1901 · 14 kg
Related groups
- EH chondrites High-iron enstatite chondrites.
- EL chondrites Low-iron enstatite chondrites.
- Aubrites Pale, almost iron-free rocks made of enstatite, from an extremely oxygen-poor world.
- K chondrites (Kakangari grouplet) A tiny grouplet with a mix of features found in no other chondrite group.
Photos shared under Creative Commons licences (Wikimedia Commons).