The effect of Al/Si disorder on the I \bar 1-P \bar 1Co-Elastic phase transition in Ca-rich plagioclase

The influence of Al/Si disorder in the anorthite tetrahedral framework upon the I \(\bar 1\) -P \(\bar 1\) displacive transition of that framework has been investigted at high-temperature by powder X-ray diffraction. The temperature-dependence of the order parameter in a heat-treated (disordered) anorthite and a Ca-rich plagioclase has been determined from spontaneous strain measurements. Both samples show appreciable disorder, with Q _od = 0.88 in both cases. In each, the critical exponent β appears to be intermediate between values for classical tricritical (1/4) and second-order (1/2) phase transitions. This critical behaviour is consistent with a Landau potential in which the coefficient of the quartic term is positive but smaller than the coefficient of the sixth order term, corresponding to a second-order phase transition close to a tricritical point. There does not appear to be any defect strain tail near T _c and inhomogeneities in Q _od appear to be on rather a short length scale in these samples. The role of changing Q _od appears to be more important than that of changing composition (albite component). The data are interpreted using a model of a homogeneous field due to changing Q _od which renormalizes the transition temperature, T _c ^* , and the fourth order coefficient, B _eff, in the Landau expansion. The results are consistent with classical Landau behaviour, and demonstrate the care which must be taken in interpreting apparently non-classical critical exponents for phase transitions close to a tricritical point.