Fig. 18

Phase boundaries of reactions involving addition of either aqueous SiO₂ or gaseous H₄SiO₄ and removal of Ca in the form of dissolved Ca bicarbonate, Ca²⁺ + 2HCO₃⁻, at X_CO2 defined by R28 (wollastonite-calcite-tilleyite equilibrium) and different values of SiO₂ activity and H₄SiO₄ fugacity defined by reactions R5 and R11 (Fig. 19). In each plot, the labeled curves of different colors separate the “primary” (above the curves) and “secondary” (below the curves) assemblages which correspond to the reactants and products of each reaction, respectively. The gray calcite saturation line separates two different phase assemblages containing silicates + calcite + aqueous solution above the line and silicates + aqueous solution below it. The tilleyite and spurrite saturation curves show conditions when a mineral starts to crystallize from an aqueous solution according to a reaction Ca²⁺ + HCO₃⁻ + SiO₂ → xCaO∙ySiO₂∙zCO₂ + CO_2(g) + H₂O_(g)