Lawrencite
Lawrencite is iron chloride, long blamed for making meteorites rust and crumble, though its role is now doubted.
Lawrencite is the mineral name for iron(II) chloride, (Fe,Ni)Cl2, named after the 19th-century American chemist J. Lawrence Smith, who studied meteorites. For well over a century, collectors blamed it for the familiar problem of iron meteorites that "sweat" droplets of brownish liquid, flake and eventually fall apart. The story was that lawrencite inside the meteorite absorbed moisture from the air, producing acidic, chloride-rich solutions that ate away at the metal.
Modern studies tell a more complicated story. Chloride is indeed central to meteorite corrosion, but much or most of it seems to come from the Earth environment, such as salty soils and groundwater, rather than from the meteorite itself. The main chloride-bearing corrosion product actually found is usually akaganeite, an iron oxyhydroxide containing chlorine, and whether lawrencite occurs as an original meteorite mineral is uncertain.
Irons that spent a long time in salty ground, including some pieces of Campo del Cielo from Argentina, are known for this kind of corrosion. Signs include greenish or brown droplets, orange flaking and cracks. Keeping specimens very dry is the best protection.
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