Armalcolite is a mineral that was discovered at Tranquility Base on the Moon by the Apollo 11 crew in 1969. It was named for Armstrong, Aldrin and Collins, the three Apollo 11 astronauts. Examples of this mineral were later discovered in various locations on Earth as well, such as Smoky Butte, Garfield County, Montana, USA in lamproite dikes and plugs; Disko Island, (Greenland) in native iron; Pripyat Swell (around the Pripyat River, Ukraine) in trachybasalt and in South African kimberlites.

Armalcolite has a chemical formula of (Mg,Fe++)Ti2O5. It has a Mohs hardness of 6 and a specific gravity of 4. It forms grey sub to anhedral masses with a crystal structure that is orthorhombic prismatic. Armalcolite is a part of the pseudobrookite group which consists of minerals of the general formula X2YO5. X and Y are usually Fe (2+ and 3+), Mg, Al, and Ti. End members are armalcolite (MgFeTi2O5), pseudobrookite (Fe2TiO5), and ferropseudobrookite (FeTi2O5).