A note on the chemistry of seawater in the range 350°-500°C

The chemistry of seawater at conditions of 350° to 500°C, 220 to 1000 bars (22 to 100 MPa) is controlled by reactions involving magnesium hydroxide sulfate (MHSH) and anhydrite. During progressive heating from 350° to 500°C at 1000 bars (100 MPa), MHSH with a  ratio of 1.25 is formed via precipitation from solution and via reaction of solution with pre-existing anhydrite. During adiabatic expansion the MHSH extracts additional SO₄ from seawater and converts to a stoichiometry in which . These reactions control and greatly change the concentrations of Ca, Mg, SO₄ in solution and produce significant ionizable hydrogen, attaining 11.7 mmoles kg⁻¹ at maximum conditions.