Pyrrhotite data

[Tom]

Pyrrhotite is an iron sulfide mineral with a non-stoichiometric composition as Fe1-xS

x varies from 0 (FeS) to 0.125 (Fe7S8) (Belzile et al., 2004).

The llnl.dat database includes information for FeS (or troilite), which is apparently rare on Earth but more common in meteorites.

Pyrrhotite
        FeS +1.0000 H+  =  + 1.0000 Fe++ + 1.0000 HS-
        log_k           -3.7193
   -delta_H   -7.9496   kJ/mol   # Calculated enthalpy of reaction   Pyrrhotite
#   Enthalpy of formation:   -24 kcal/mol
        -analytic -1.5785e+002 -5.2258e-002 3.9711e+003 6.3195e+001 6.2012e+001
#       -Range:  0-300


The sit.dat database includes information for Fe0.875S (x = 0.125, ie Fe7S8). This phase is often reported as monoclinic pyrrhotite.

Pyrrhotite
Fe0.87S = +0.870Fe+2     -1.000H+     +1.000HS-     -0.260e-     
     log_k    -5.59         #
     delta_h   2.45         kJ/mol        #
     # Enthalpy of formation:             -97.5         kJ/mol        95ROB/HEM


I have identified compositions for pyrrhotite towards the Fe7S8 composition, from a pyrrhotite-pentlandite ore in Sudbury, Canada.

If I can find appropriate thermodynamic and log k data I am keen to add some different pyrrhotite compositions to a custom database.

Does anyone have any tips on database modification/addition or know of useful resources for thermodynamic data? I will be looking into the literature.

I'm guessing I can just define a new phase as "pyrrhotite2"...









Reference:

Belzile, N et al. (2004). Journal of Geochemical Exploration. 84, 65–76.

[dlparkhurst]

There is also the phase Mackinawite in the wateq and phreeqc databases, but I think it tends to form in rapidly precipitating sulfidic environments.

Adding a new phase is fairly straightforward, just note that the log K for Fe7S8 is going to be 8 times that for Fe0.875S.

[rfembilejr]

Hi David,

I have a pyrrhotite chemical analysis data as below:
               wt%
Fe      59.86
Ni      0.79
Cu      0.10
S      38.05

I wanted to add a new phase using the formula that I got (Fe0.986Ni0.012S)
but not sure about the products I should use on the right side of the equation.
I initially came up with the following based on Tom's reaction for Fe0.87S.

Fe0.986Ni0.012S = +0.986Fe+2 + 0.012Ni+2   -1.000H+   + 1.00HS-   -0.004e-

Does this look correct?

[dlparkhurst]

From llnl.dat the simple definition of pyrrhotite is

Pyrrhotite
        FeS +1.0000 H+  =  + 1.0000 Fe++ + 1.0000 HS-
        log_k           -3.7193

I would simply replace part of the iron with nickel, but accounting for charge balance. If you are doing inverse modeling, the log K is not used, but is needed for reaction modeling.

My_Pyrrhotite
        Fe0.986Ni0.0142S +1.0000 H+  =  + 0.986 Fe++ + 0.014Ni++ +1.0000 HS-
        log_k           -3.7193