Description: As laboratory experiments predict, a Mg-hydroxide-sulfate-hydrate mineral, here named caminite, precipitates in nature from seawater heated in an active submarine hydrothermal system. Caminite is found intergrown with anhydrite in the wall of a black-smoker chimney precipitated around hydrothermal fluids discharging on the East Pacific Rise axis at 2l'N latitude. Caminite is tetragonal (space group I4,/amd) with a = 5.239Å and c = 12.988Å. The five strongest lines appearing in X-ray powder difraction patterns (CuKa radiation) are 3.345 (I/Io = 100; hkl = 103); 3.220 (80; 112); 1.871 (50; I l6); 1.620 (25; 303); 1.609 (20; 224). Bond-strength calculations and experimental results predict that the caminite structure accommodates a range of compositions described by a general formula: MgSO4 . xMg(OH)2 . (1- 2x)H2O, where 0 〈 x 〈 0.5. The caminite in our sample has a composition corresponding to a stoichiometry of MgSO4 . 0.4Mg(OH)2 . 0.2H2O. It is soft (H = 2.5) and apparently colorless. Caminite is uniaxial negative and has low birefringence (0.002). Its indices of refraction are omega = 1.534 and epsilon = 1.532. In the recharge zones of submarine hydrothermal systems, large volumes of convecting seawater heated above approximately 240°C rnay precipitate abundant caminite and anhydrite. Formation of abundant caminite can drastically lower the pH of downwelling seawater in such systems, and rapid removal of sulfate into caminite and anhydrite may prevent the reduction of much seawater sulfate to sulfide within the hydrothermal system. Incorporation of seawater sulfate into caminite and anhydrite at elevated temperatures and subsequent recycling of this sulfate into the oceans by dissolution at low temperatures should affect the oxygen-isotope compositionof seawater sulfate and may play a part in maintaining the oxygen-isotope values of oceanic sulfate in disequilibrium with δO18 of seawater.