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A Landau potential with linear-quadratic coupling has been developed to describe interactions between a non-convergent order parameter, Q OD , for Fe/Mg ordering, and the order parameter, Q D , for the C2/c-P2 1 /c phase transition in pigeonite. Spontaneous strain relationships, and expressions for the effect of ordering on the transition temperature derived from this expansion, have been tested by single crystal X-ray diffraction methods. Lattice parameters collected from a natural pigeonite crystal with composition En 47 Fs 44 Wo 9 , in situ at temperatures up to 1050 °C, reveal that increasing Q OD  could act to suppress Q D  by a mechanism which includes overlapping and opposing strain fields. In a second experiment, the intensities of superlattice reflections (h + k = 2n + 1) were followed in situ at temperatures up to 500 °C. The crystal was heated ex situ successively at 700, 750, 800, and 850 °C between repeated in situ measurements in order to produce changes in the degree of cation order. The resulting data sets, giving the temperature dependence of Q D  2  for different fixed values of Q OD , are consistent with the initial Landau model. In particular, they show a strong and linear dependence of transition temperature on Q OD . The fourth order coefficient of the expansion describing the phase transition is perhaps also renormalized by changes in Q OD . It is suggested that the influence of Q OD  on the phase transition could be greater than the influence of the phase transition on the equilibrium variation of Q OD .

Coupling between non-convergent ordering and transition temperature in the C2/c ↔ P2 1 /c phase transition in pigeonite