Apollo 16 rocks contain rustlike alteration. A mass of rust-colored, iron-rich crystals forming a grain 30 µm in diameter in rock 66095 has been identified by x-ray diffraction as akaganéite (β FeOOH). Other altered polycrystalline masses of less diameter are found associated with native FeNi metal grains.
Akaganéite is known to occur in meteorite specimens, resulting from the oxhydration of lawrencite (FeCl2) and native FeNi upon exposure to water vapor. The occurrence of akaganéite in the Apollo 16 rocks is essentially the same as in meteorites, and a similar process of alteration in an oxidizing environment is posulated for its origin. All or at least most of the akaganéite formed as a direct result of contamination by terrestrial water vapor of FeCl2-bearing lunar rocks.