1. Balance chemical reactions. Start by choosing a species to appear on the left side of the reaction. Move to the Results pane. Rxn writes a reaction with Pyrite on the left side, balanced by the default basis species Rxn will automatically add its default set of species.

2. Fe(OH) 3 (ppd) has been swapped in for Fe ++ O 2 (g) swapped in for O 2 (aq) Recast the reaction by performing basis swaps. Click to swap out an entry, then choose a species to replace it The reaction is automatically rebalanced in terms of a ferric iron mineral and O 2 (g).

3. Rxn can calculate equilibrium constants at any temperature Log K at the specified temperature Log Ks for the above reaction at the principal temperatures

4. Choose pH and Eh units You can use Rxn to calculate equilibrium lines, such as those appearing on activity diagrams. Start by balancing the reaction Set temperature Constrain the activity of species in the reaction.

5. Click to expand output Equation for equilibrium line is written in terms of Eh and pH

6. Set the water activity to model equilibrium in a brine. You can use Rxn to calculate the temperature at which a reaction is in equilibrium. Do not set temperature Reaction for the dehydration of gypsum

7. Anhydrite, a dehydrated mineral, is the stable phase above 11.8°C in the brine.