Reinjection of formation waters creates few chemical problems if no large change in temperature has occurred, no gas or vapor has separated, access of air has been prevented, and the fluids have had an opportunity to react with the minerals to the point of compatibility. Injection of incompatible fluids may cause chemical problems. There is a need for prediction of fluid-mineral reactions.

Most geochemists who have worked on mineral-fluid reactions have had inadequate data and have been reduced to assuming reaction states. The state most geochemists have assumed is one of equilibrium because it is uniquely defined and readily calculable. In an effort to test the applicability of an equilibrium model under real geologic conditions, real systems have been studied. Systems are chosen where reactions are known to occur and where the reactant minerals and fluids and product minerals and fluids can be identified and analyzed. Very seldom have stable equilibrium (minimum Gibbs free energy) conditions been found, although in some instances metastable equilibrium conditions have been found. The assumption of a stable equilibrium state is a very poor choice of model, especially a temperatures of 100°C and below.

The general lack of attainment of equilibrium in no way impairs prediction of reactions, however. Admittedly there is no a prior thermodynamic method of predicting what phase will react, nor can the necessary departures from equilibrium be predicted before a reaction will occur at a significant rate, because thermodynamic arguments are time-independent. However, in geologic systems now accessible, enough reactions are occurring that, by observation, an empirical knowledge will provide a base for predicting reactions.

The equations used for describing reactions states are

[EQUATION]

and

[EQUATION]

where

^DgrG_R is the Gibbs free energy of the reaction
R is the gas constant
T is the temperature in degrees Kelvin
Q is the reaction quotient
K is the equilibrium constant
n is the number of electrons in redox reactions
F is the volt-gram equivalent
Eh_eq is the Eh that would be measured if the reaction were at equilibrium
Eh_m is the measured Eh.

^DgrG_R is the exact statement of the departure from equilibrium for the reaction of interest.

Reactions should be studied in both reaction directions. Most reactions are asymmetric in that the ^DgrG_R's required to dissolve most phases are of different values than the ^DgrG_R's necessary to form the mineral from solution. The reactions should all be treated as congruent reactions. All the species in solution generated by complete solution of the solid must be considered. Using incongruent reactions introduces the unwarranted assumption of equilibrium.

Some generalizations about the result can be made. Small, highly charged cations yield hydrous metastable phases (Fe(OH)₃, amorphous silica) or a stable phase (Mg(Oh)₂) at very slight supersaturations. Subsequent dehydration is very sluggish. Simple anhydrous carbonates require greater departures from equilibrium for precipitation to occur, but dissolve fairly readily. Simple anhydrous silicates dissolve with modest unsaturation but the anhydrous crystalline silicates require large supersaturations for precipitation. Sulfide minerals, although they may dissolve with oxidation of the sulfur to SO₄^-2, do not form except with enormous supersaturations. The problem is probably with the bisulfide ion. Siderite requires much less departure from eq ilibrium than pyrite for precipitation, yet both have Fe⁺² as the cation.

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