A Structural Explanation of the Polymorphism and Transitions of $$MgSiO_{3}$$

Differences between the polymorphs of MgSiO3 consist essentially of different ways of stacking slabs of $$SiO_{3}$$ chains, and transitions between the polymorphs may be effected by movements of chains by two-thirds of the z-axis spacing, together with associated displacements of Mg atoms by one-third of c. The transitions from proto to rhombic enstatite and from proto to clino enstatite involve the same percentage of displaced atoms, but, because the displaced atoms are distributed more uniformly in the second transition, it is thought that a nucleus of clino enstatite will propagate more easily than one of rhombic enstatite. This suggestion is consistent with the rapid metastable formation of clino enstatite at low temperatures and with the sluggish formation of rhombic enstatite (often very disordered) from proto enstatite. Shearing stress should favor the formation of clino enstatite in conformity with the experiments of Turner et al., and thus it may be a "stress mineral" in the sense of Harker. Highly complex schemes for arranging the SiO3 chains are possible, and, as an example, three possible sequences are proposed for the enstatite with a 36 Å a-axis described by Byström.