The phase transition in annealed samples of K-bearing cordierite (K0.2Mg2[Al4.2Si4.8O18])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 (Qod) 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.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.