SPONTANEOUS STRAIN AND ORDER PARAMETER COUPLING ASSOCIATED WITH THE INCOMMENSURATE PHASE TRANSITION IN NEPHELINE

The lattice parameters of a natural sample of nepheline (~Na3KAl4Si4O16) have been determined from high resolution neutron powder diffraction patterns. Data were collected during heating from 4 to 650K and during cooling from 630 to 15 K. During the heating cycle disordering of potassium atoms and vacancies occurs, but during cooling the incommensurate structure returns by a purely displacive mechanism. As expected for incommensurate phase transitions, the changes in lattice parameters are small but, thanks to the exceptionally high resolution of HRPD at the ISIS facility, they are clearly resolved for the first time. K/vacancy ordering causes a shearing of the structure (contraction along c and expansion along a), while the purely displacive transition causes a small volume increase. The volume strain at 4 K is ~1.6‰, and the precision is significantly better than this. The disordering transition has Tc ~ 450K and the cooling data are consistent with Tc ~ 300 K for the purely displacive transition, though the latter needs to be obtained from independent observations.

Because displacive and order/disorder components of the incommensurate phase transition cause some strain, it is likely that they can couple by a common strain mechanism. The coupling will be bilinear, with the implication that the main effect will be to renormalise the transition temperature. The low temperature, displacive transition will occur at higher temperatures if cooling is slow enough to allow K/vacancy ordering to occur. This interpretation provides a basis for developing a Landau model of the phase transition, in which bilinear coupling occurs between incommensurate structures formed by a displacive mechanism and by an order/disorder mechanism. The same coupling mechanism is likely to be responsible for the incommensurate structure of plagioclase feldspars, and we predict the existence of a displacive incommensurate phase transition in intermediate plagioclases with high degrees of Al/Si disorder.