an iron arsenide mineral (FeAs₂) that usually occurs with iron and copper sulfides in hydrothermal vein deposits. It typically occurs with impurities of cobalt, nickel, and arsenic—as at the Andreas-Berg, in the Ore Mountains (Erzgebirge) of Germany; Andalusia, Spain; and Franklin, New Jersey, U.S. Loellingite is classified in a group of iron, cobalt, and nickel arsenides that form orthorhombic crystals and are called sulfide minerals, though they contain very little sulfur. See sulfide mineral for further information.