R chondrite
R chondrites are highly oxidised, metal-poor chondrites rich in iron-bearing olivine, with distinctive oxygen isotopes.
Only one observed fall gives the R chondrites, or rumurutiites, their name: Rumuruti, which fell in Kenya on 28 January 1934 and was recognised as the first R fall in the 1990s. Earlier finds such as Carlisle Lakes from Western Australia had already hinted at the group. R chondrites are among the most oxidised chondrites: iron-nickel metal is nearly absent, and much of the iron sits in olivine with a fayalite content around Fa38 to Fa40, often making up about 70 per cent of the rock. Sulfides such as pyrrhotite and pentlandite are prominent.
Bulk compositions resemble ordinary chondrites in some respects, but oxygen isotopes set R chondrites clearly apart, with Δ17O around +2.5 to +3 per mil, the highest of any chondrite group. Most are regolith breccias with light and dark fragments, solar-wind gases and petrologic types from 3 to 6 mixed together. Rare members contain hornblende or mica, minerals that need water, recording unusual high-temperature fluid activity.
Almost all R chondrites are finds from hot deserts and Antarctica. Recognition requires mineral analysis showing iron-rich olivine and virtually no metal, and bulk oxygen-isotope measurements.
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