Pyroxenes are single-chain silicates with the general formula XYSi2O6, where X and Y are either both divalent cations (mainly Ca, Fe, Mg), or mono-(Na, Li) and trivalent cations (Al, Fe) respectively. The stacking of chains gives rise to two types of cavities, labeled M1 (close to octahedral in shape, where the Y cations are located) and M2 (more irregular in shape, where the X cations are located). The relative rotation of the Si-centred tetrahedra about the bridging oxygen atom is responsible for the structural flexibility of pyroxenes and explains their polymorphism and their existence in a wide range of temperature and pressure environments. The different polymorphs of pyroxenes can be described as derived from the stacking of modules consisting of layers contained within pairs of (100) planes located on tetrahedral cations.