CH chondrites
'High metal' carbonaceous chondrites with tiny chondrules.
- Classification
- Stony meteorites › Chondrites › Carbonaceous chondrites › CH chondrites
- Share of witnessed falls
- none yet seen to fall
- In our catalogue
- 44 meteorites
- Heaviest
- Isheyevo, 16 kg
- Type specimen
- Allan Hills 85085, found in Antarctica in 1985
CH chondrites are rich in iron-nickel metal, about 20% of the rock by volume, and have very small chondrules, typically only a few hundredths of a millimetre to about 0.1 mm across. The first recognised was Allan Hills 85085 from Antarctica, and none has yet been seen to fall. All are petrologic type 3, with some hydrated fragments.
Chemically they belong with the CR and CB chondrites. Many of their chondrules may have formed in a vapour plume from a giant collision between planetesimals rather than in the solar nebula.
Mineralogy and chemistry
CH stands for 'high metal'. Besides tiny chondrules, mostly cryptocrystalline (glassy-looking, with crystals too fine to see) or skeletal, CH chondrites contain small CAIs and metal grains that are often chemically zoned, with nickel and cobalt rising from core to rim as if they condensed directly from hot gas. There is almost no fine matrix; instead the rock holds small clasts of hydrated material. The metal shows nickel-cobalt ratios close to the Sun's, and the bulk rock is depleted in volatile elements such as sodium and zinc.
Their oxygen isotopes lie on the line shared with CR and CB chondrites, below the terrestrial line. Like CRs, they carry nitrogen with a strong excess of nitrogen-15.
Parent body and formation
CH chondrites are thought to be mixtures: some ordinary nebular chondrules and CAIs, plus a large population of tiny chondrules and zoned metal grains that condensed in a hot, dense plume of gas from a high-speed impact between planetesimals, about 5 million years after the first solids. The same event is thought to have made the CB chondrites. Isheyevo, found in Russia in 2003, contains both CH-like and CB-like layers and links the two groups.
Abundance and examples
Several dozen CH chondrites are known, all finds, mostly from the Sahara, the Arabian deserts and Antarctica. Allan Hills 85085 is the type specimen, and Acfer 182 from Algeria is one of the larger and better studied. With no witnessed fall, they make up none of the fall statistics.
Why they matter
The CH and CB chondrites are the best evidence that some chondrules formed by collisions between planetesimals rather than in the quiet disc. If so, by about 5 million years after the first solids, some bodies were already large and hot enough to produce incandescent plumes when they collided. The chondrules in CHs thus record a violent stage of planet building, when growing bodies smashed into each other at several kilometres per second.
For collectors, CH chondrites are among the rarest of all chondrites, and most known specimens are small. Many have been recognised only since the 1990s, as systematic searches in hot deserts began to produce large numbers of stony meteorites for study.
How to recognise one
A cut face shows a dense scattering of metal grains in a dark rock where chondrules are hardly visible to the eye. CHs are strongly magnetic and heavy for a stone. They can be confused with metal-rich ordinary chondrites or with CBs, so microscopic study of chondrule size and metal zoning is essential.
Famous examples
- Allan Hills 85085 CH3, Antarctica, 1985 · 12 g
- Acfer 182 CH3, Algeria, 1990 · 166 g
- Isheyevo CH/CBb, Russia, 2003 · 16 kg
Related groups
- CB chondrites (bencubbinites) More than half metal, with chondrules made in a planet-sized collision.
- CR chondrites Metal-bearing carbonaceous chondrites with some of the most pristine organic matter known.
- Carbonaceous chondrites Rare, often dark chondrites rich in carbon, water-bearing minerals and organic compounds, including the building blocks of life.
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