The binary phase diagram in "Fig. 2" shows the phases of a mixture of two substances in varying concentrations, formula_1 and formula_2. The region labeled "formula_3" is a solid solution, with formula_2 acting as the solute in a matrix of formula_1. On the other end of the concentration scale, the region labeled "formula_6" is also a solid solution, with formula_1 acting as the solute in a matrix of formula_2. The large solid region in between the formula_3 and formula_6 solid solutions, labeled "formula_3 + formula_6", is "not" a solid solution. Instead, an examination of the microstructure of a mixture in this range would reveal two phases—solid solution formula_1-in-formula_2 and solid solution formula_2-in-formula_1 would form separate phases, perhaps lamella or grains.