The influence of A1/Si disorder in the anorthite tetrahedral framework upon the I1-P1 displacive transition of that framework has been investigted at high-temperatme by powder X-ray diffraction. The temperaturedependence of the order parameter in a heat-treated (disordered) anorthite and a Ca-rich plagioctase has been determined from spontaneous strain measurements. Both samples show appreciable disorder, with Qoa = 0.88 in both cases. In each, the critical exponent/~ appears to be intermediate between values for classical tricritical (]) and second-order ( 89 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~ and inhomogeneities in Qoa appear to be on rather a short length scale in these samples. The role of changing Qoa 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 Qoa which renormalizes the transition temperature, T*, and the fourth order coefficient, Bell, 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.