Sanidine ($\mathrm{KAlS}{\mathrm{i}}_3{\mathrm{O}}_8$)

A high-temperature potassium feldspar mineral, with idealized composition $\mathrm{KAlS}{\mathrm{i}}_3{\mathrm{O}}_8$. Substitution on its K and Al sites changes the thermodynamic activity of that mineral endmember and therefore the chemical equilibrium of reactions involving it.