But carbonates also can be formed when there is a lot of carbon dioxide dissolved in water, which causes carbonates to precipitate out. Magnetite is a compound of iron and oxygen that also is produced by bacteria, as well as many other organisms, such as whales, dolphins, bees and even humans. As the name suggests, the mineral is magnetic. Many researchers believe that bacteria, and even larger animals, use a small particle of magnetite in their bodies as a sense organ to help them navigate in the Earth's magnetic field. When a cluster of bacteria die, the magnetite particles can clump together, eventually forming large aggregations called lodestones.
Iron sulfide is similar to magnetite, but less magnetic, and is used by bacteria in the same way.
Both can be formed without the presence of life, but require quite different conditions. Magnetite is formed when a lot of oxygen is present, while iron sulfide is formed when there is no oxygen. The two could not form in nature at the same time without having been produced by living organisms. PAHs are found throughout the universe. They are produced whenever carbon-containing molecules are heated to high temperatures, such as when gasoline is burned in a car engine, or coal is burned in a power plant or a steak is cooked on the grill.