This paper deals with the Fe⁺⁺ / Mg ratios in coexisting pyroxenes and in coexisting olivines and calcium-poor pyroxenes. Theoretical relations between these ratios are investigated by use of thermodynamic functions. The slopes of tie lines defined by the distribution of magnesium and iron in analyzed pyroxenes and olivines also are employed to predict the degree of ideality of solid solutions of the metasilicates of calcium, magnesium, and iron.