EH chondrites
High-iron enstatite chondrites.
- Classification
- Stony meteorites › Chondrites › Enstatite chondrites › EH chondrites
- Share of witnessed falls
- about 0.8%
- In our catalogue
- 170 meteorites, 9 seen to fall
- Heaviest
- Abee, 107 kg
- Earliest recorded fall
- Indarch, 1891
- Most recent fall
- Itqiy, 1990
EH chondrites have more metal and smaller chondrules than EL chondrites, and contain niningerite, a magnesium-iron sulphide. Abee, which fell in Canada in 1952, is a famous EH impact breccia of about 107 kg.
Mineralogy and chemistry
EH chondrites have about 30% total iron by weight, more than any ordinary chondrite group, with roughly 20% to 25% of the rock as metal and sulphide. Their kamacite contains about 2% to 3.5% silicon. The key sulphide is niningerite, a magnesium-rich sulphide, alongside troilite, oldhamite and daubréelite. Chondrules are small, around 0.2 mm, and abundant in the least heated examples.
Their enstatite contains almost no iron. Rare minerals include perryite (a nickel-iron silicide), the potassium-iron sulphide djerfisherite and the silicon oxynitride sinoite. EH chondrites are richer than EL chondrites in volatile elements such as sodium and selenium. Like all enstatite chondrites, their oxygen isotopes plot on Earth's line.
Parent body and formation
EH chondrites come from a parent body separate from the EL chondrites, as shown by their different bulk chemistry and mineral thermometry. Most are petrologic types 3 to 5; EH6 examples are rare. Many show signs of shock and of impact melting, and some meteorites classified as EH impact-melt rocks crystallised entirely from melt produced in collisions. Their minerals record formation in conditions so reduced that they are unlike almost any rock on Earth.
Abundance and famous examples
EH chondrites are about 0.8% of falls. The group is named for its chemistry, so there is no type specimen. Abee fell near Abee, Alberta, on 9 June 1952 as a single 107 kg stone and is a breccia of EH4 fragments. Indarch, which fell in Azerbaijan in 1891 (27 kg), is a classic EH4. Qingzhen, which fell in Guizhou, China, in 1976, is an unusually primitive EH3 that still contains presolar grains of silicon carbide and diamond. Kota-Kota from Malawi and Sahara 97096 are other well-studied EH3 chondrites.
Clues to Earth's origin
EH chondrites are the meteorites most often compared with the building blocks of Earth, because their isotopes of oxygen, nitrogen, chromium, titanium and many other elements match Earth closely. Computer models of Earth's growth that start from EH-like material can reproduce several features of the planet, including its large metal core, though they need extra magnesium to match the mantle. Their very low oxygen content would also have made a large share of Earth's silicon go into the core, a question still being tested in laboratory experiments at core pressures.
For these reasons enstatite chondrites have attracted special attention in studies of the early Earth and the Moon, which shares the same oxygen isotope signature.
How to recognise one
An EH is a dark grey, metal-rich stone with small chondrules and a strong pull on a magnet. Fresh surfaces show bright metal and bronze sulphides. It weathers rapidly, with rust spreading from the metal, and its sulphides are sensitive to water, so specimens should be kept dry, ideally sealed with a drying agent such as silica gel.
Famous examples
- Abee EH4, Canada, 1952 · 107 kg
- Indarch EH4, Azerbaijan, 1891 · 27 kg
- Qingzhen EH3, China, 1976 · 2.6 kg
- Kota-Kota EH3, Malawi, 1905 · 334 g
- Sahara 97096 EH3, 1997 · 2.52 kg
Related groups
- EL chondrites Low-iron enstatite chondrites.
- Enstatite chondrites Chondrites formed with almost no oxygen to spare, full of rare minerals unknown on Earth's surface.
- Aubrites Pale, almost iron-free rocks made of enstatite, from an extremely oxygen-poor world.
Photos shared under Creative Commons licences (Wikimedia Commons).