Germanium in Icelandic geothermal systems

Germanium concentrations in geothermal waters in Iceland lie mostly in the range 2–30 ppb. There is an overall positive relation between the germanium content of the water and its temperature. Most of the germanium occurs as Ge(OH)^?₅in solution but Ge(OH)₄ may also be present in significant amounts in saline waters when above 200°C. Evidence indicates that aqueous germanium concentrations are controlled by exchange reactions where it substitutes for silica in silicates and iron in sulphides. It is the rate of dissolution and the relative abundance of the alteration minerals which take up germanium to a variable extent that ultimately fix Ge(OH)₄ concentrations in the water. This, together with water pH, fixes total dissolved germanium. It is mostly the primary rock composition that dictates the relative abundance of the alteration minerals. Conductive cooling in upflow zones favours removal of germanium from solution. During the initial stages of boiling of rising hot water dissolution is enhanced but precipitation at later stages.Thermodynamic data of various aqueous germanium species and several minerals are summarized and dissociation constants and solubilities estimated at elevated temperatures using available predictive methods.