Need help creating new database entries for a variety of Na-rich minerals

[Jeremy Boak]

I have been trying to model the mineral stability of a number of sodium phases found in the Eocene Green River Formation of Colorado, Utah, and Wyoming. The minerals are nahcolite (NaHCO3), Natron (Na2CO3), and Trona (Na2CO3•NaHCO3•2H2O). Data for all three are available, although in different databases. Once I copied some files across to a single database, I plotted the stability of the three phases, and compared it to recent data from Jagniecki, and others (Jagniecki, E.A., Lowenstein, T.K., Jenkins, D., and Demicco, R. V., 2015, Eocene atmospheric CO₂ from the nahcolite proxy. Geology. v. 43, p. 1075-1078, doi:10.1130/G36886.1.).  The two results show significantly different locations for the triple point at 1 atm total pressure, as shown below. The formation of large quantities of nahcolite in the Piceance Basin of Colorado at temperatures (from fluid inclusions) of 21-28°C are reasonable given the new triple point (the colored lines, where the gold line shows the reaction between nahcolite and trona at halite saturation. The triple point from existing data in GWB databases would require extreme CO2 activities equivalent to >3000 ppm atmospheric just to stabilize nahcolite at the low end of that temperature range. I would like to create new database entries using the data in the supplement to the cited paper. However, I am not especially well versed in the appropriate methods, and my thermodynamic training is more than 40 years behind me. I attach a table of various thermodynamic data from Robie and Hemingway (1995), from several of the available GWB databases, as well as from Jagniecki and others, 2015 (who cite Robie and Hemingway for nahcolite data).

I would also be interested in constructing similar database entries for analcime solid solutions of the formula NaAl (x)Si(3-x)O6•((3-x)/2)H2O. My last student had worked with another committee member to generate some of these, based upon the van't Hoff equation, but I find that these appear to reduce the stability of analcime from the entry in GWB, with a value of x=0.96, whereas data on Green River Formation analcime indicate values of x ranging from 0.75-0.95. The relative stability of the Na saline mineral above and authigenic Na-minerals analcime and dawsonite are important to understanding the evolution and character of Lake Uinta in which they were formed. Any help would be much appreciated.

NahcoliteTronaNatron Thermo data.xlsx

[Jia Wang]

Hello,

A good place to start is to calculate the free energy change for the reaction of interest using the delta H and S provided in the supplementary document for your reaction. This value will depend on the species in the reaction. Once you have the change in free energy, you can solve for the equilibrium constant at standard temperature (25C) using the equation delta0G = -RT*ln(K_eq).  

I believe a similar thread discussing this and can be helpful for you: https://forum.gwb.com/topic/2106-glass-chemistry/

I would discourage copying and pasting data entries across datasets unless you are sure that the entry is consistent with the activity model intended for destination dataset. For example, thermo_freezechem.tdat and thermo_coldchem.tdat uses the Harvie-Moller-Weare formalism of the Pitzer equations to calculate activity coefficients. thermo.tdat and thermo.comV8.R6+.tdat are datasets that use the Bdot activity model. Therefore, copying and pasting entries across the datasets may result in very erroneous results. You can find the activity model for each dataset at the top of the file (if you open it in a text editor) or on the Header pane (if you view the dataset using TEdit). 

Another thing to note is that the log K format for each dataset may be different. For some datasets published, a six term polynomial is used to express the log equilibrium constants within a temperature range. For example, thermo_freezechem and thermo_coldchem both give their log k values as polynomial expansions. The coefficient to fit the polynomial is given for the entry. You should note that these coefficients are not the direct log K (e.g. Natron Na2CO3•10H2O from freezechem in your excel spreadsheet). If you would like to see the log Ks at the principal temperature specified in the Tables tab, you can go to the Header pane and switch the log K format from polynomial to table. TEdit will calculate the log K at those temperatures for you. 

Hope this helps,
Jia Wang
Aqueous Solutions LLC

 

[Jeremy Boak]

Jia Wang - Thank you much for this response.  I have been away from the site for a while on other tasks. I hope I can implement this approach when I get some time to work on it.