frezchem

Legacy repository of the FREZCHEM code developed by Giles Marion (Desert Research Institute) et al. This and previous versions used to be freely accessible at frezchem.dri.edu but that page is no longer accessible.

Efforts are underway by Jonathan Toner and David Catling at the University of Washington to revisit the thermodynamic model behind FREZCHEM and move it to the versatile PHREEQC code. Here is an example publication from their work. A FREZCHEM-like PHREEQC implementation is provided as the three PHREEQC_* files above (freezing routine, thermodynamic database, output processing pipeline).

Below are a description of the PHREEQC freezing routine as well as some of the release notes for FREZCHEM version 17. See full v17 release notes.

PHREEQC freezing routine

PHREEQC freezing routine (PHREEQC_Frz.txt) emulating the capabilities of FREZCHEM, with a more restricted (Na-Mg-K-Ca-SO4-Cl) but thermodynamically consistent chemical space (see Toner & Catling 2017). This is a PHREEQC input file to be used in combination with the Toner & Catling database augmented with molar volumes (PHREEQC_TC17_vm.txt). See comments within PHREEQC_Frz.txt about how to switch between equilibrium and fractional crystallization, and between freezing (temp stepping at BASIC l. 21) and evaporation (water stepping).

Outputs are saved in a ./Out folder and require bash concatenating prior to plotting using PHREEQC_Pipeline.xslx. In a terminal window on Mac or Linux, this can be done automatically from within the Out folder by typing:

find . -type f -name "*.txt" |
while read file; do
    sed -n '2{p;q;}' $file >> z.out
done

or in a single command line:

find . -type f -name "*.txt" | while read file; do     sed -n '2{p;q;}' $file >> z.out; done

The contents of the generated z.out file can be pasted directly in the first tab ("Raw") of PHREEQC_Pipeline_v3.xslx. Sort this tab by descending temperature (column D, high to low) for the plots in the third "Processed" tab to look good.

This routine has been benchmarked against Toner & Catling (2017) Tables 1 and 2, as well as against Marion et al. (2005) Fig. 4 & 5.

FREZCHEM release notes (v17)

Attached is a "beta" version of the FREZCHEM model that includes:

• chloride, bromide, perchlorate, nitrate, sulfate, and bicarbonate-carbonate salts,
• strong acid chemistry,
• ferrous and ferric iron chemistry,
• aluminum and silicon chemistries,
• ammonia and ammonium chemistries,
• methane, ethane, and propane chemistries, and
• gas hydrate chemistry.

This version includes both temperature and pressure dependencies. This repository includes a FORTRAN source code file, four input files, and a PDF file including these instructions as well as a list of chemical species in the model (Table 1), instructions for model inputs (Tables 2-5, 8, and 10), and four examples of model outputs (Tables 6-7, 9 and 11).

General advice

This model is very much a work in progress. I [Giles Marion] may be adding new chemistries to the model in the next few years. I have not spent much time debugging the model or making it user-friendly. In addition, there are convergence problems, at times, with the model. My [Giles Marion's] version of the model was created with Absoft's ProFortran for Mac. Porting this code to another FORTRAN compiler is always problematic. (Note from Marc: I use the freely available GFortran compiler). Once you have a FREZCHEM model working, verify correctness from an example from published data (see examples in the PDF file). If you have additional problems, email me (Giles Marion). Indicate the FREZCHEM version you are using (e.g., FREZCHEM17) and your model input.

The model is an equilibrium chemical thermodynamic model, meaning it will always select the most stable minerals. There are a few minerals (e.g., aragonite and vaterite) that are always metastable with respect to other minerals (e.g., calcite). To explicitly include a metastable mineral in your calculations necessitates removing the stable mineral from the mineral database. This is most simply done by assigning the stable mineral an arbitrary high Ksp through SOLIDPHASE.txt. The # of the Ksp for a specific mineral in the FORTRAN program is the same as the solid phase # in Table 1 of the PDF file (e.g., K52 is the solubility product for calcite). If you are using the model to calculate pH, then you should make sure that the initial solution is charge-balanced. Otherwise, the model will force a charge balance by changing the bicarbonate-carbonate or acid concentrations, which could lead to a serious error in calculated pH if the solution is badly charge-balanced. If necessary, force a charge-balance in the initial solution by changing a major constituent that minimizes the effect on pH (e.g., Na or Cl). If you input Fe, Al, Si, or alkalinity, then you will have four options on how to deal with pH; adding NH3-NH4 adds a fifth option (see Table 2 of the PDF file).

Compared to earlier versions, this Version 17 of the FREZCHEM model contains new parameterizations dealing with ethane, propane, methane, and nitrogen species. Also, Versions 15-17 lowered the temperature level to 173 K (see Tables 8-11 of the PDF file).

Model inputs

A fundamental change was made in FREZCHEM 13 on how to input data into the model. Earlier versions required inputs via the computer screen. Versions 13-17 require inputs via data files; this approach simplifies and speeds up model inputting. There are four input files that must be built to run FREZCHEM. In the PDF file:

• Table 2 describes the main model inputs (material in the Input.txt file of Table 3)
• Table 3 presents the main Input.txt file. Inputs are all placed to the left of the ,. Do not remove the ,. That comma separates model input from descriptive words.
• Table 4 describes in more detail how to handle gases in Input.txt
• Table 5 lumps togeher three minor input files: SOLIDPHASE.txt, SOLIDMASS.txt, and NUANCES.txt.
  • SOLIDPHASE.txt (Table 5A) allows the user to remove all solid phases from equilibrium calculations or some specific minerals. That option allows for a pure solution phase calculation without any minerals precipitating. In the molar to molal conversion example in Table 7 of the PDF file), all solid phases had to be removed. Note the assigned exceptionally high equilibrium constants in Table 7, which is what keeps the solid phases from precipitating. In a previous Phoenix site calculation (Marion et al., 2010), we removed magnesite (MgCO3) and dolomite [CaMg(CO3)2] from model calculations (Table 5A), which led to calcite (CaCO3) and hydromagnesite [3Mg(CO3)2•MgSO4•3H2O] precipitating. Generally, model calculations start with aqueous/gas phases, without initial solid phases; but if you want a particular solid phase to control the solution phase chemical composition, then you can specify this solid phase in SOLIDPHASE.txt.
  • SOLIDPHASE.txt (Table 5B) is where the molar amounts of the phases in SOLIDPHASE.txt are specified. These are arbitrary amounts that must not completely dissolve in 1.0 kg H2O. For example, we assumed that Earth seawater would have been saturated with dolomite during Snowball Earth (Marion and Kargel, 2008). Changing the first line of SOLIDMASS.txt from No(0) to YES(1) and specifying dolomite would allow saturation with dolomite.
  • NUANCES.txt (Table 5C) allows for temperature, water content, or pressure changes to be adjusted during a specific run. For example, if you want to know the eutectic temperature of a salt assemblage, and you know that this will occur slightly below 259 K, you could change the ∆T term from 5 K between 298 and 263 K (as assigned by Input.txt) to 1 K between 263 and 259 K, and 0.1 K below 259 K. This scenario would allow for a more accurate estimate of the eutectic temperature than using either a 5 K or 1 K term for the ∆T decrement. With respect to NUANCES.txt, always retain two steps for temperature, water content, and pressure changes, even if you need to duplicate two steps (e.g., 263.15 0.1, 263.15 0.1).

Input.txt

• Title: Any alphanumeric character up to 50 characters.
• Freeze(1) or Evaporation(2) or Pressure (3) Pathway: Enter 1, 2, or 3 depending on whether you want to simulate a temperature change (1), an evaporation (2), or a pressure change (3). For evaluating a single point, enter 1.
• Equilibrium(1) or Fractional(2) Crystallization: In equilibrium crystallization (1), precipitated solids are allowed to re-equilibrate with the solution phase as environmental conditions change. In fractional crystallization (2), precipitated solids are removed and not allowed to re-equilibrate with the solution phase as environmental conditions change.
• Seawater Salinity: If you want seawater salinity to govern the calculations, enter 1 for Yes, and 0 for No.
• Practical Salinity: If Yes in the above line, enter SP. If No, enter 0.0.
• Calcite Supersaturation in Seawater: If you want this to be considered, enter 1 for Yes, or 0 for No.

Cations

Sodium (m/kg): Enter sodium molality (moles/kg(water)). Otherwise, enter 0.0. Same for Potassium, Calcium, Magnesium, Strontium, Ferrous Iron, Ferric Iron, Aluminum, Silicon, and Ammonium.

Acidity and pH

If Iron, Aluminum, Silicon, Alkalinity, or Ammonia-Ammonium molalities are nonzero, then choose an acidity option:

• Acidity ignored (Option 1), enter 1.
• Acidity fixed by pH (Option 2), enter 2.
• Acidity fixed by H+ ion concentration (Option 3), enter 3.
• Acidity fixed by alkalinity (Option 4), enter 4.
• Acidity fixed by NH3(aq) and NH4(aq) (Option 5), enter 5.
• Initial pH: If an acidity opttion was chosen, then for Option 1, enter 0; Option 2, enter the pH; Options 3 and 4, enter an approximate pH; Option 5, enter 10. If no acidity option was chosen, specify the pH. (e.g., 7.0).

Anions

• Chloride (m/kg): Enter chloride molality (moles/kg(water)). Otherwise, enter 0.0. Same for Bromide, Perchlorate, Sulfate, and Nitrate.
• Carbonate Alkalinity: Enter as equivalents/kg(water). If alkalinity = 0.0, then you must enter 0.0. The latter will cause the model to skip all bicarbonate-carbonate with pH chemistries in the model.
• Sulfite Alkalinity: Enter as equivalents/kg(water). If alkalinity = 0.0, then you must enter 0.0.
• Sulfide Acidity: Enter as equivalents/kg(water). If acidity = 0.0, then you must enter 0.0.
• Acidity: Enter as equivalents/kg(water). This is the total hydrogen concentration, if known initially. Generally this is only known for strong acid solutions. For example, for a 1 molal H2SO4 solution, enter 2.00. Otherwise, enter 0.0. The equations used to calculate pH for the alkalinity and acidity cases are incompatible. So, a specification of either carbonate alkalinity or acidity requires that the other variable be assigned a value of 0.0. This will channel the calculations to the proper algorithm.
  • HCl(bars): If the HCl atmospheric concentration is known, then specify here. Otherwise, enter 0.0. If you specify 0.0, then the model will calculate HCl(bars). Note that if you specify HCl(bars) or the other acids below, then these properties override the total acidity specification (see above). That is, the solution is equilibrated with the atmospheric concentration. You can specify atmospheric concentrations for some acids (e.g., HCl and HNO3) and leave other acid partial pressure unspecified (e.g., H2SO4 = 0.0).
  • HNO3(bars): If the HNO3 atmospheric concentration is known, then specify here. Otherwise, enter 0.0.
  • H2SO4(bars): If the H2SO4 atmospheric concentration is known, then specify here. Otherwise, enter 0.0.
• Boron (m/kg): Enter boron molality (moles/kg(water)). Otherwise, enter 0.0.
• Fluoride (m/kg): Enter fluoride molality (moles/kg(water)). Otherwise, enter 0.0.

Gases

• Initial Total Pressure (bars): Enter the initial total pressure of the system.
• Initial CO2(bars): If alkalinity > 0.0 or CO2 hydrates are simulated, then specify the initial concentration of CO2(g) in bars.
• Mole Fraction of CO2: Enter the mole fraction of CO2(g) for the system (mole fraction = PCO2/total pressure). For pure CO2, enter 1.0. If 0.0, then CO2(g) is fixed and independent of total pressure. O2(bars): If the atmospheric concentration of oxygen is known, then specify here. Otherwise, enter 0.0. If you are interested in ferrous iron chemistry, then you may want to assign O2 a value of 0.0. Otherwise, it is likely that the insolubility of ferric minerals in the presence of O2 will cause all the iron to precipitate as a ferric mineral (see discussions in Marion et al., 2003a).
• Initial CH4(bars): If CH4 is simulated, then specify the initial concentration of CH4(g) in bars.
• Mole Fraction of CH4: Enter the mole fraction of CH4(g) for the system (mole fraction = PCH4/total pressure). For pure CH4, enter 1.0. If 0.0, then CH4(g) is fixed and independent of total pressure.
• Initial NH3(bars): If NH3(g) are inputs, then specify the initial concentration of NH3(g) in bars.
• Initial NH3(aq): Enter NH3(aq) molality (moles/kg(water)). Do not enter positive values (> 0) for both NH3(bars) and NH3(aq).
• Initial N2(bars): If the atmospheric concentration of nitrogen is known, then specify here. Otherwise, enter 0.0.
• Mole Fraction of N2: Enter the mole fraction of N2(g) for the system (mole fraction = PN2/total pressure). For pure N2, enter 1.0. If 0.0, then N2(g) is fixed and independent of total pressure.

Gas hydrates

Mixed CH4-CO2 Gas Hydrate?: If both CH4(g) and CO2(g) are specified as inputs, then you can use this data to estimate the stability of a mixed CH4-CO2 gas hydrate (1) or treat the two gases as independent gas hydrates (0). Same for oher combinations of N2, CH4, CO2, C2H6, and C3H8.

Other inputs

• Molar to Molal Conversion?: If you want to convert molar data into molal concentrations, then enter 1 for Yes or 0 for No.
• Salinity/liter: If "Yes" above, then you must enter the total aqueous salinity (g salt/liter), which can be calculated from molar data [g salt/liter = ∑(moles/liter) x (g salt/mole)]. In the case depicted in Table 7 of the PDF file, the SL value is 316.57 g salt/liter (5.417 x 58.44).
• Initial Temperature(K): Enter the temperature Kelvin for start of simulation (e.g., 273.15).
• For Temperature Change Pathway(1):
  • Final Temperature(K): Enter final temperature of simulation (e.g., 263.15).
  • Temperature Decrement(K): The temperature interval between simulations (e.g. 1). For the above temperature designations, the model would calculate equilibrium starting at 273.15 K and ending at 263.15 K at 1 K intervals. If you want to change the decrement in a run (e.g., to reduce the step size near an equilibrium), see File NUANCES.txt.
• For Evaporation Pathway(2):
  • Initial Water (g): Normally enter 1000. The standard weight basis of the model is 1000 g water plus associated salts. In you enter 100 instead, the initial ion concentrations specified above will be multiplied by 10 (i.e., 1000/100). This feature of the model is useful in precisely locating where minerals start to precipitate during the evaporation process without having to calculate every small change between 1000 g and 1 g.
  • Final Water (g): Enter the final amount of water that you want to remain in the system (e.g., 100).
  • Water Decrement (g): Enter the water decrement for simulations (e.g., 50 g). Specifying initial = 1000, final = 100, and decrement = 50 would result in calculations at 1000 g, 950 g, ... 100 g. To change the decrement in a run (e.g., to reduce the step size near an equilibrium), use NUANCES.txt.
• For Pressure Pathway(3):
  • Final Pressure(bars): Enter the final pressure of the simulation [e.g., 101.01325 bars (100 atm)].
  • Pressure Increment(bars): Enter the pressure increment. For example, if initial pressure is 1.01 bars, final pressure is 101.01 bars, and pressure increment is 1.0 bars, then the simulation would calculate at 1.01, 2.01, 3.01, ... 101.01325 bars. To change the increment in a run, use NUANCES.txt.

Model outputs

See Table 6 of the PDF file.

• Temp(K): temperature.
• Ion.Str. is the ionic strength of the equilibrium solution (see Table 6 of the PDF file).
• RHO is the density of the solution.
• Phi is the osmotic coefficient of the equilibrium solution.
• H2O(g) is the amount of water remaining as liquid.
• Ice is the amount of water that formed ice between 273.15 K. The mass basis for calculation in the model is 1.0 kg of water (except for evaporation); therefore, the water in liquid water + ice + hydrated salts should always sum to 1.0 kg.
• Press.(bars): pressure.

Solution section

• Initial Conc. are the input concentrations at 273 K (Table 3).
• Final Conc. are the equilibrium concentrations at 273 K.
• Act. coef. (activity coefficient) and Activity are self-explanatory.
• Moles are the # of moles in the solution phase.
• Mass Balance: for the major constituents, this column should generally agree with the input column (Initial Conc.). This is the best check on the internal consistency of the calculations.

Solids section

• Moles: final number of moles present.
• Equil. Constant: equilibrium constant.
• Accum Moles: net number of moles of that solid that have precipitated.

Example simulations using the new features of recent versions

Table 6 of the PDF file is a case where we examined sodium/chloride/ethane/propane (Marion et al., 2015) using the Input.txt file in Table 3 of the PDF file. The high concentrations of ethane and propane cases causes a solid phase of these gases forming together on a mixed gas hydrate. See the bottom of Table 6 with both ethane and propane forming together.

Table 7 is a case where we [Marion et al.] converted molar into molal concentrations. In the upper table there are three columns labeled rho, SA, and CF that represent model calculated estimates of density (kg(soln.)/liter or g/cm3), absolute salinity [g salt/kg(soln.)], and the conversion factor [liters/kg(H2O)]. The iterations quickly converted molar concentrations (5.4170 moles/liter under Initial Conc.) into molal concentrations [6.1458 moles/kg(H2O) under Final Conc.]. In addition to inputs of molar concentrations, this algorithm also requires salinity on a liter basis (SL) (see Table 3). In this case, the SL value is 316.57 g/liter [= 5.417 x 58.44 (molecular weight of NaCl)]. So even with no prior knowledge about solution density (the model arbitrarily assigns initial density = 1.00 g/cm3), we were able to quickly calculate density and convert molar to molal concentrations. In turn, molal concentrations could be directly imported into FREZCHEM to explore geochemical processes. Note that all the potential solid phases were assigned high solubility products to prevent their precipitation (Table 7). FORTRAN model inputs to accomplish this negation of solid phases are in the SOLIDPHASE.txt file in Table 5 and must be implemented by the user. See Marion (2007) for a fuller discussion of the techniques used in this algorithm.

Table 8 is an input file for a Titan simulation from 273 K to 173 K that is dominated with NH3(aq), NH3(g), NH4(aq), and CH4(g).

Table 9 is the output of this simulation after the initial temperature at 273 K dropped to 173 K. At 173 K, ice had formed and methane hydrate, NH4Cl, and (NH4)2SO4 had precipitated. NH3(aq), which started at 10.0 molal, has risen to 36.27 molal that is approaching the eutectic where NH3•2H2O would precipitate. In this case, the pH started at 10.0 (Table 8) and rose to 18.03 (Table 9). The latter may not be accurate.

Table 10 is an input file for a Titan simulation from 273 to 173 K that is dominated with NH3(aq), NH3(g), NH4(aq), N2(bars), and CH4(bars).

Table 11 is the output of this simulation after the initial temperature at 273 K dropped to 173 K. At 173 K, ice had formed, NH4Cl and (NH4)2SO4 had precipitated, and N2•6H2O-CH4•6H2O had formed. NH3(aq), which started at 10.0 molal, has risen to 30.34 molal. In this case, the pH started at 10.0 (Table 10) and rose to 17.89. These Tables 10-11 are very similar to Tables 8-9, except that a mixed gas hydrate formed in this case and methane hydrate formed in the previous case.

Publications

The validation of this model is discussed in 17 publications:

1. Spencer et al. (1990) The prediction of mineral solubilities in natural waters: A chemical equilibrium model for the Na-K-Ca-Mg-Cl-SO4-H2O system. Geochim. Cosmochim. Acta 54:575-590. https://doi.org/10.1016/0016-7037(90)90354-N
2. Marion and Farren (1999) Mineral solubilities in the Na-K-Mg-Ca-Cl-SO4-H2O system: A re-evaluation of the sulfate chemistry in the Spencer-Møller-Weare model. Geochim. Cosmochim. Acta 63:1305-1318. https://doi.org/10.1016/S0016-7037(99)00102-7
3. Marion (2001) Carbonate mineral solubility at low temperatures in the Na-K-Mg-Ca-H-Cl-SO4-OH-HCO3-CO3-CO2-H2O system. Geochim. Cosmochim. Acta 65:1883-1896. https://doi.org/10.1016/S0016-7037(00)00588-3
4. Marion (2002) A molal-based model for strong acid chemistry at low temperatures (<200 to 298 K). Geochim. Cosmochim. Acta 66:2499-2516. https://doi.org/10.1016/S0016-7037(02)00857-8
5. Marion et al. (2003) Modeling aqueous ferrous iron chemistry at low temperatures with application to Mars. Geochim. Cosmochim. Acta 67:4251-4266. https://doi.org/10.1016/S0016-7037(03)00372-7
6. Marion et al. (2005) Effects of pressure on aqueous chemical equilibria at subzero temperatures with applications to Europa. Geochim. Cosmochim. Acta 69:259-274. https://doi.org/10.1016/j.gca.2004.06.024
7. Marion et al. (2006) Modeling gas hydrate equilibria in electrolyte solutions. Calphad 30:248-259. https://doi.org/10.1016/j.calphad.2006.04.002
8. Marion (2007) Adapting molar data (without density) for molal models. Computers & Geosciences 33:829-834. https://doi.org/10.1016/j.cageo.2006.10.009
9. Marion and Kargel (2008) Cold Aqueous Planetary Geochemistry with FREZCHEM: From Modeling to the Search for Life at the Limits. Springer. https://doi.org/10.1007/978-3-540-75679-8
10. Marion et al. (2008) Modeling ferrous-ferric iron chemistry with application to Martian surface geochemistry. Geochim. Cosmochim. Acta 72:242-266. https://doi.org/10.1016/j.gca.2007.10.012
11. Marion et al. (2009) Br/Cl partitioning in chloride minerals in the Burns formation on Mars. Icarus 200:436-445. https://doi.org/10.1016/j.icarus.2008.12.004
12. Marion et al. (2009) Modeling aluminum-silicon chemistries and application to Australian playa lakes as analogues for Mars. Geochim. Cosmochim. Acta 73:3493-3511. https://doi.org/10.1016/j.gca.2009.03.013
13. Marion et al. (2011) Modeling hot spring chemistries with applications to Martian silica formation. Icarus 212:629-642. https://doi.org/10.1016/j.icarus.2011.01.035
14. Marion et al. (2012) Modeling ammonia-ammonium aqueous chemistries in the Solar System’s icy bodies. Icarus 220:932-946. https://doi.org/10.1016/j.icarus.2012.06.016
15. Marion et al. (2013) Sulfite-sulfide-sulfate-carbonate equilibria with applications to Mars. Icarus 225:342-351. https://doi.org/10.1016/j.icarus.2013.02.035
16. Marion et al (2014) Modeling nitrogen-gas, -liquid, -solid chemistries at low temperatures (173-298 K) with applications to Titan. Icarus 236:1-8. https://doi.org/10.1016/j.icarus.2014.03.025
17. Marion et al (2015) Modeling nitrogen and methane with ethane and propane gas hydrates at low temperatures (173-290 K) with applications to Titan. Icarus 257:355-361. https://doi.org/10.1016/j.icarus.2015.04.035