Geothermal mineral equilibria

The dominant reaction determining the chemistry of fluids in a geothermal system of the New Zealand type consists of the conversion of primary plagioclase by CO₂ to calcite and clays with log pco₂ = 15.26 ? 7850/(t + 273.2), temperature t in °C. Subsequent reactions involving secondary minerals control relative CO₂-H₂S-contents. The distribution of mineral phases throughout a geothermal system reflects the stepwise conversion of thermodynamically unstable primary phases through a series of intermediate, metastable phases to a thermodynamically stable, secondary assemblage. The relative stabilities of these phases was evaluated on the basis of their solubilities, the least soluble aluminiumsilicate representing the thermodynamically most stable phase under a given set of conditions. Observed assemblages of secondary minerals in geothermal systems represent indicators allowing mineral/fluidinteraction conditions to be evaluated on the basis of multi-component mineral stability diagrams.