The Birth of a Black Smoker: The key to the creation of black smoker chimneys is an unusual chemical property of the mineral anhydrite, or calcium sulfate (CaSO4). Unlike most minerals, anhydrite dissolves at low temperatures and precipitates at high temperatures greater than ~150?C (302?F).
Left: 350?C hydrothermal fluid (containing dissolved Ca2+ ions) exits the seafloor and mixes with cold seawater (containing dissolved Ca2+ and SO42- ions). A ring of anhydrite precipitates around a vent opening and the jet of hot fluid.
Middle: Hydrothermal fluid and seawater mix through the nascent anhydrite wall around the vent opening. Sulfide and sulfate minerals precipitate within pore spaces of the wall, which gradually becomes less permeable. The anhydrite wall also provides a substrate, or foothold, on which other minerals can precipitate.
Right: Different minerals precipitate at different temperatures. Chalcopyrite is deposited against the inner wall, which is adjacent to 350?C hydrothermal fluid. Anhydrite, iron, copper-iron, and zinc-sulfide minerals (such as pyrite, marcasite, bornite, sphalerite, wurtzite) precipitate at lower temperatures within pore spaces of the chimney wall.