CK chondrites
Heated, highly oxidised carbonaceous chondrites, cousins of the CVs.
- Classification
- Stony meteorites › Chondrites › Carbonaceous chondrites › CK chondrites
- Share of witnessed falls
- about 0.2%
- In our catalogue
- 229 meteorites, 2 seen to fall
- Heaviest
- Karoonda, 41.73 kg
- Earliest recorded fall
- Karoonda, 1930
- Most recent fall
- Kobe, 1999
- Type specimen
- Karoonda, fell in South Australia in 1930
CK chondrites, named after Karoonda in Australia, are unusual among carbonaceous chondrites because most were strongly heated, to petrologic types 4 to 6. They are rich in magnetite and contain almost no metal, a sign of very oxidising conditions.
Their chemistry and oxygen isotopes overlap with CV chondrites, and some researchers think CV and CK meteorites come from the same parent asteroid, with CKs from its hotter interior.
Mineralogy and chemistry
CK chondrites are made mostly of olivine rich in iron (about 29–33 mol% fayalite), with plagioclase, calcium-rich pyroxene and a striking amount of magnetite, often containing chromium and grains of exsolved sulphide and spinel. Metal is almost absent. Many CKs have 'darkened' silicates, peppered with tiny grains of magnetite and sulphide, which makes the rock dark grey even though it was strongly heated.
Chondrules are about 0.7 to 1 mm across, and CAIs are present but less common than in CVs. Most CKs are petrologic types 4 to 6; a few type 3 examples are known. Their oxygen isotopes plot below the terrestrial line and overlap those of CV chondrites.
Parent body and formation
CKs record the most oxidising conditions and the highest temperatures of any carbonaceous chondrite group, reaching several hundred degrees Celsius. Their close match to CVs in oxygen isotopes, chondrule sizes and refractory inclusions has led many researchers to propose that the two groups come from a single parent asteroid, with CVs from cooler outer layers and CKs from its hotter interior, though some differences in chemistry keep the question open.
Abundance and famous examples
CKs are rare among falls, about 0.2%. Karoonda, the type specimen, fell in South Australia on 25 November 1930, and about 41 kg was recovered. Karoonda itself is a CK4. Most other CKs are finds from Antarctica and the Sahara. Because they look so unlike most carbonaceous chondrites, CKs were recognised as a separate group only in 1991.
Recent finds
Most known CKs have been found since the 1980s, first by teams searching the Antarctic ice and later by collectors in the deserts of north-west Africa. Some recently recognised stones are intermediate between CV and CK, with CV-like inclusions but CK-like heating and magnetite. These transitional meteorites are central to the question of whether the two groups share a parent. If they do, the CV–CK parent would be a rare example of a carbonaceous asteroid with a hot, metamorphosed interior.
CKs have also been compared with some dark asteroids whose spectra show the absorption of magnetite, but no parent body has been identified.
How to recognise one
CKs are dark grey to black rocks with a fairly uniform, recrystallised texture in which chondrules are hard to see. Abundant magnetite makes them clearly magnetic despite having almost no metal, so they can be confused with ordinary chondrites. They are firm rather than crumbly, and fresh falls have a black fusion crust much like any other stony meteorite. Laboratory study of the olivine, magnetite and oxygen isotopes is needed to confirm one.
Famous examples
- Karoonda CK4, Australia, 1930 · 41.73 kg
Related groups
- CV chondrites Large chondrules and big white inclusions, the oldest solids in the Solar System. Allende is the classic.
- R chondrites (rumurutiites) Highly oxidised chondrites with almost no metal.
- Carbonaceous chondrites Rare, often dark chondrites rich in carbon, water-bearing minerals and organic compounds, including the building blocks of life.
Photos shared under Creative Commons licences (Wikimedia Commons).