Alabandite (MnS) Geochemistry:Eh-pH diagram-Solubility-Stability Field

Alabandite is a sulfide mineral often found in epithermal sulfide vein deposits. The name of the mineral is derived from its supposed discovery locality at Alabanda, Aïdin, Turkey.

• Chemical Formula:     MnS
• Composition:     Molecular Weight = 87.00 gm
• General physical properties and color photograph of Alabandite can be found at http://www.mindat.org/min-89.html
• Google Image also has tons of great Alabandite photographs.

Today, I am just going to do some “educational” tasks using Geochemist’s Workbench. Let’s begin with a solubility reaction. Alabandite is highly unstable in oxidized environment and breaks down to Mn+2 and SO4– rapidly. The oxidation of Alabandite can be presented by the simple reaction:

• Alabandite  + 2 O2(aq)  = Mn++  + SO4–

• Log K’s:

0 °C:  152.1275        150 °C:   89.7573
25 °C:  137.9632        200 °C:   76.7436
60 °C:  121.2437        250 °C:   65.5350
100 °C:  105.6272        300 °C:   55.4212

• Polynomial fit:  log K = 152.1 – .6062 × T + .001732 × T^2 – 3.537e-6 × T^3 + 3.056e-9 × T^4

• Equilibrium equation for Alabandite: log K = log a[Mn++] + log a[SO4–] – 2 × log a[O2(aq)] ————————–(1)

Like many other minerals, the log K for Alabandite gets smaller with higher temperature.  So, Alabandite is more soluble at higher temperature. Geochemist’s Workbench only allow calculations upto 300 degree centigrade. Using the polynomial fit, we can plot the log K of Alabandite vs temperature. Figure 1 shows the log K curve.

Stability Diagrams:

Now I am going to present some stability diagrams. Lets start with a simple Tempeature-log f(O2) diagram. I am using a .001 as activity for Alabandite. Now crystallized MnSO4 can also be formed by the oxydation of Alabandite.

Alabandite  + 2 O2(aq)  = MnSO4(c)

• Log K’s:

0 °C:  148.5194        150 °C:   91.3790
25 °C:  135.2914        200 °C:   79.9353
60 °C:  119.8758        250 °C:   70.4165
100 °C:  105.6417        300 °C:   62.4797

Polynomial fit:
log K = 148.5 – .5684 × T + .001736 × T^2 – 3.806e-6 × T^3 + 3.829e-9 × T^4

Equilibrium equation:
log K = – 2 × log a[O2(aq)]

Notice about the difference between equilibrium reactions.

So, the diagram is showing that at higher temperature the reaction needs less oxygen to proceed.

Some more stability field diagrams:

This diagram tells you that at a very reducing condition (low Eh), Mn will be stable as solid  phase alabandite. In  highly oxidized environment Pyrolusite will be the most stable phase.

Alabandite  + 2.5 O2(aq)  + H2O  = Pyrolusite  + SO4–  + 2 H+

Just enjoy the different Eh-pH diagrams that shows different solid and soluble phases of Mn. Notice how the MnSO4 becomes crystalline at higher temperature.