Crystal formations such as this grow out of mineral-rich solutions in spacious rock cavities. When the solution becomes saturated, crystals begin to form on rock walls, on other crystals, or even on particles of dust. They grow quickly or slowly--or stop--depending on changes in temperature and the concentration of the solution. Marcasite forms from iron and sulfur to make crystals with a chemical composition of FeS2. Note how the size of the forming crystals grew larger with each round of formation in this specimen.