In the simple system CaSiO3-FeSiO3-MgSiO3, pyroxene M1 and M2 sites can be treated as distinct but interpenetrating solutions. In order to study the thermodynamics of the cation exchange between these sites, the simple mixture model is used as a first approximation of their nonideality. The intracrystalline cation distribution of iron and magnesium in synthetic pigeonite predicted in this way agrees well with experimental data.