Iddingsite
Iddingsite is a reddish mixture of clay and iron oxides formed when water altered olivine, notably in Martian nakhlites.
Iddingsite is not a single mineral but a fine-grained reddish-brown mixture of clays, iron oxides and other products that forms when olivine is attacked by water. It is named after the American geologist Joseph Iddings.
The nakhlites, meteorites from Mars, contain veins of iddingsite cutting through their olivine crystals. Dating of these veins and their contents suggested the alteration took place on Mars about 600 to 700 million years ago or later, long after the rock crystallised. Some veins are cut by the meteorite's fusion crust, which proves they formed before the rock arrived on Earth rather than during its time in the ground.
Iddingsite in nakhlites is therefore direct evidence that liquid water existed on Mars at times when the planet was already cold and dry. Its tiny clay crystals and carbonates have been studied in great detail to learn the temperature and chemistry of that Martian water.
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