R chondrites (rumurutiites)
Highly oxidised chondrites with almost no metal.
- Classification
- Stony meteorites › Chondrites › R chondrites (rumurutiites)
- Share of witnessed falls
- about 0.1%
- In our catalogue
- 135 meteorites, 1 seen to fall
- Heaviest
- Northwest Africa 7514, 23.62 kg
- Earliest recorded fall
- Rumuruti, 1934
- Type specimen
- Rumuruti, fell in Kenya in 1934
R chondrites are named after Rumuruti in Kenya, the only one seen to fall (1934). They are rich in iron-bearing olivine and nickel-rich sulphides and contain almost no free metal, showing they formed in more oxidising conditions than ordinary chondrites. Their oxygen isotopes are among the heaviest of any meteorites.
Most are breccias of mixed petrologic types. Rare examples contain amphibole and mica, minerals that need water to form.
Mineralogy and chemistry
R chondrites are about 70% olivine, and that olivine is rich in iron, about 37–40 mol% fayalite, more than in any ordinary chondrite. Metal is almost absent; iron and nickel are held instead in sulphides such as pyrrhotite and pentlandite. Chondrules average about 0.4 mm and make up less than half the rock, with abundant matrix.
Their oxygen isotopes are the most positive of any chondrite group, with a Δ17O of about +2.7 per mil, well above the terrestrial line. Bulk chemistry resembles ordinary chondrites in many elements but with less metal and more volatile elements, a sign that they formed from similar ingredients under much more oxidising conditions.
Parent body and formation
Most R chondrites are regolith breccias, cemented from fragments of different petrologic types and carrying solar-wind gases from the surface of their parent asteroid. Some heated R chondrites contain hornblende-like amphibole and mica, minerals that need water, showing that fluids were present deep inside the body; LaPaz Icefield 04840 from Antarctica is the best example. The parent body is unknown. The group was first recognised from Carlisle Lakes, a find from Western Australia, and was originally called the Carlisle Lakes grouplet.
Abundance, type specimen and examples
Rumuruti, which fell in Kenya on 28 January 1934, is the type specimen and the only R chondrite seen to fall; only 67 g was recovered. Several hundred R chondrites are now known, almost all finds from North Africa, Oman and Antarctica. Northwest Africa 753 is one of the larger and better studied.
Related groups
R chondrites resemble ordinary chondrites in their chondrule sizes and in many trace elements, so they may have formed from similar material in the inner Solar System. Their high oxidation state and heavy oxygen isotopes, however, suggest that water or oxidised ice played a part. Some researchers have compared them with the CK chondrites, the other highly oxidised, magnetite- and sulphide-rich group, though the two have very different oxygen isotopes and come from different parent bodies.
The heated R chondrites containing amphibole and mica are rare examples of chondrites holding these water-bearing minerals, which on Earth typically form where hot fluids pass through rock deep underground.
How to recognise one
R chondrites are fine-grained, often brecciated stones, light to dark grey when fresh but usually brownish from weathering of their sulphides. With almost no metal, they barely react to a magnet and show no metal flecks on a cut face, which can mislead hunters into thinking they are not meteorites. Laboratory analysis of olivine and oxygen isotopes is needed.
Famous examples
- Rumuruti R3.8-6, Kenya, 1934 · 67 g
- Carlisle Lakes R3.8, Australia, 1977 · 50 g
- Northwest Africa 753 R3.9, 2000 · 12 kg
- LaPaz Icefield 04840 R6, 2004 · 50 g
Related groups
- Ordinary chondrites The everyday meteorite: four in every five falls. Three groups, H, L and LL, from at least three asteroids.
- CK chondrites Heated, highly oxidised carbonaceous chondrites, cousins of the CVs.
- Chondrites Primitive stones that never melted, made of the first solids of the Solar System. The most common meteorites of all.
See all 135 R chondrites (rumurutiites)
Photos shared under Creative Commons licences (Wikimedia Commons).