Phlogopite ($\mathrm{KM}{\mathrm{g}}_3\mathrm{AlS}{\mathrm{i}}_3{\mathrm{O}}_{10}(\mathrm{OH}{)}_2$)

The magnesian biotite mineral endmember. Iron substitution on its magnesium-bearing positions produces a solid solution requiring an activity model when used in chemical equilibrium calculations.