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This keyword data block is used to define a solid-solution assemblage. Each solid solution may be nonideal with two components or ideal with any number of components. The initial amount of each component in each solid solution is defined in this keyword data block. Any calculation involving solid solutions assumes that all solid solutions dissolve entirely and reprecipitate in equilibrium with the solution. The formulation is sufficiently general that synthetic organic liquid solutions also can be simulated.
number --A positive number designates the following solid-solution assemblage and its composition. A range of numbers may also be given in the form m-n , where m and n are positive integers, m is less than n , and the two numbers are separated by a hyphen without intervening spaces. Default is 1.
-comp --Identifier indicates a component of an ideal solid solution is defined. Component is part of the solid solution defined by the preceding Line 1. Optionally, comp , component , or -c [ omponent ].
phase name --Name of the pure phase that is a component in the solid solution. A phase with this name must have been defined in a PHASES data block.
-comp1 --Identifier indicates the first component of a nonideal, binary solid solution is defined. The component is part of the solid solution defined by the preceding Line 1. Optionally, comp1 or -comp1 .
phase name --Name of the pure phase that is component 1 of the nonideal solid solution. A phase with this name must have been defined in a PHASES data block.
-comp2 --Identifier indicates the second component of a nonideal, binary solid solution is defined. The component is part of the solid solution defined by the preceding Line 1. Optionally, comp2 or -comp2 .
phase name --Name of the pure phase that is component 2 of the nonideal solid solution. A phase with this name must have been defined in a PHASES data block.
-temp --Temperature at which excess free-energy parameters are defined, in Celsius. Temperature, either temp or tempk , is used if excess free-energy parameters are input with any of the following identifiers: -gugg_nondim , -activity_coefficients , -distribution_coefficients , -miscibility_gap , -spinodal_gap , -alyotropic_point , or -margules . Optionally, temp , tempc , or -t [ empc ]. Default is 25 °C.
-tempk --Temperature at which excess free-energy parameters are defined, in kelvin. Temperature, either temp or tempk , is used if excess free-energy parameters are input with any of the following options: -gugg_nondim , -activity_coefficients , -distribution_coefficients , -miscibility_gap , -spinodal_gap , -alyotropic_point , or -margules . Optionally, tempk or -tempk . Default is 298.15 K.
-critical_point --The mole fraction of component 2 at the critical point and the critical temperature (kelvin) are used to calculate dimensional Guggenheim parameters. Optionally, critical_point or -cr [ itical_point ].
-alyotropic_point --The mole fraction of component 2 at the alyotropic point and the total solubility product at that point are used to calculate dimensional Guggenheim parameters. Optionally, alyotropic_point or -al [ yotropic_point ].
Multiple solid solutions may be defined by multiple sets of Lines 1, 2, 3, and 4. Line 2 may be repeated as necessary to define all the components of an ideal solid solution. Nonideal solid solution components must be defined with Lines 3 and 4. Calculations with solid solutions assume that the entire solid recrystallizes to be in equilibrium with the aqueous phase. This assumption is usually unrealistic because it is likely that only the outer layer of a solid would re-equilibrate with the solution, even given long periods of time. In most cases, the use of ideal solid solutions is also unrealistic because nonideal effects are nearly always present in solids. Liquid solutions of synthetic organic liquids usually behave as ideal mixtures and can be modeled well with this keyword (Appelo and Postma, 2005, Chapter 10, Example 10.5).
Lines 7-16 provide alternative ways of defining the excess free energy of a nonideal, binary solid solution. Only one of these lines should be included in the definition of a single solid solution. The parameters in the Example data block are taken from Glynn (1991) and Glynn (1990) for “nondefective” calcite (log K -8.48) and dolomite (expressed as Ca 0.5 Mg 0.5 CO 3 , log K -8.545; note that a phase for dolomite with the given name, composition, and log K would have to be defined in a PHASES data block because it differs from the standard stoichiometry for dolomite in the databases). In the Example data block, Lines 7 through 16, except Line 14 (alyotropic point), define the same dimensional Guggenheim parameters. Internally, the program converts any one of these forms of input into dimensional Guggenheim parameters. When a batch-reaction or transport calculation is performed, the temperature of the calculation (as defined by mixing of solutions, REACTION_TEMPERATURE data block, or heat transport in TRANSPORT simulations) is used to convert the dimensional Guggenheim parameters to nondimensional Guggenheim parameters, which are then used in the calculation.
The identifiers -gugg_nondim , -activity_coefficients , -distribution_coefficients , -miscibility_gap , -spinodal_gap , -alyotropic_point , or -margules define parameters for a particular temperature which are converted to dimensional Guggenheim parameters by using the default temperature of 25 °C or the temperature specified in Line 5 or 6. If more than one Line 5 and (or) 6 is defined, the last definition will take precedence. If -alyotropic_point or -distribution_coefficients identifiers are used to define excess free-energy parameters, the dimensional Guggenheim parameters are dependent on (1) the values included with these two identifiers, and (2) the equilibrium constants for the pure-phase components. The latter are defined by a PHASES data block in the input file or database file.
The parameters for excess free energy are dependent on which component is labeled “1” and which component is labeled “2”. It is recommended that the component with the smaller value of log K be selected as component 1 and the component with the larger value of log K be selected as component 2. The excess free-energy parameters must be consistent with this numbering. A positive value of (nondimensional Guggenheim parameter) or (dimensional Guggenheim parameter) will result in skewing the excess free-energy function toward component 2 and, if a miscibility gap is present, it will not be symmetric about a mole fraction of 0.5, but instead will be shifted toward component 2. In the calcite-dolomite example, the positive value of (1.90) results in a miscibility gap extending almost to pure dolomite (mole fractions of miscibility gap are 0.0428 to 0.9991).
After a batch reaction with a solid-solution assemblage has been simulated, it is possible to save the resulting solid-solution compositions with the SAVE keyword. If the new compositions are not saved, the solid-solution compositions will remain the same as they were before the batch reaction. Use of RUN_CELLS for a batch reaction automatically saves the new compositions of all reactants. After it has been defined or saved, the solid-solution assemblage may be used in subsequent simulations through the USE or RUN_CELLS keywords. Solid-solution compositions are automatically saved following each shift in advection and transport calculations.
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