Pyrope ($\mathrm{M}{\mathrm{g}}_3\mathrm{A}{\mathrm{l}}_2\mathrm{S}{\mathrm{i}}_3{\mathrm{O}}_{12}$)

The magnesian aluminium-bearing garnet mineral endmember. In an ideal Mg–Fe site model with other positions fixed, its thermodynamic activity is $X_{\mathrm{Mg}}^3$ because three magnesium positions occur per formula unit.