In the crystal structure of any pyroxene there are three sets of cation sites which we may designate A (coordination 6 or more), A′ (coordination 6), and B (4-coordinated), in the ratio represented by A′AB2O6, for example CaMgSi2O6 (Warren and Bragg, 1929; Warren and Modell, 1930; Morimoto, et al., 1960). For the present purpose A′ and A are not differentiated, so that the formula may be written A2B2O6. In rock-forming pyroxenes the B sites contain mainly Si, with usually some Al, and apparently occasionally a few Fe3+ and Ti ions if the B sites are not filled by the available Si and Al ions. To save repetition the ability of Fe3+ and Ti to act as Al in some circumstances will not be mentioned again. All cations other than Si may occupy the A sites. Al can thus be present in both A and B positions.