Muscovite ($\mathrm{KA}{\mathrm{l}}_3\mathrm{S}{\mathrm{i}}_3{\mathrm{O}}_{10}(\mathrm{OH}{)}_2$)

A mica mineral with an idealized endmember composition $\mathrm{KA}{\mathrm{l}}_3\mathrm{S}{\mathrm{i}}_3{\mathrm{O}}_{10}(\mathrm{OH}{)}_2$. Its formula contains one tetrahedral Al and two octahedral Al per formula unit. Under ideal independent mixing on crystallographic sites, an endmember thermodynamic activity includes all mixed sites rather than only a bulk mole fraction.