X-ray powder-diffraction and infrared study of the hexagonal to orthorhombic phase transition in K-bearing cordierite

The phase transition in annealed samples of K-bearing cordierite (K_0.2Mg₂[Al_4.2Si_4.8O₁₈])from the hexagonal form to the modulated form to the orthorhombic form has been studied by hard-mode infrared spectroscopy and by synchrotron X-ray powder diffraction. Our results reveal the behavior of (Al,Si) ordering (Q_od) and spontaneous strain or orthorhombic distortion (Q) as a function of annealing time during this time-dependent transition. Two phase transitions are observed, namely between a hexagonal and a modulated form and between a modulated and an orthorhombic form. Both transitions are strongly first order. The structural states of K-bearing cordierite appear to be very similar to the pure-Mg cordierite with the following modifications: (1) The maximum distortion of the orthorhombic structure leads to a strain corresponding to a Δ index of Δ = 0.17 as compared to Δ = 0.25 in pure-Mg cordierite. (2) The kinetics of the transformation process are accelerated by a factor of about two by doping with K. A thermodynamic description of the transition behavior leads to a form for the Landau potential in which the effect of K in the structure is identified as a strain field conjugate to the order parameter.