Changing lattice metrics of synthetic cordierites; the metastable hexagonal to orthorhombic transformation sequence during isothermal annealing

Starting from glasses heated at 1290 degrees C, the irreversible transition of initially formed high-cordierite to orthorhombic low- cordierite was studied for pure Mg (sub 2) Al (sub 4) Si (sub 5) O (sub 18) as well as for K-doped cordierite according to the scheme KAlSi (sub -1) with 0.10 and 0.25 K pfu, using the Guinier method and synchrotron powder XRD, respectively. Instead of the classical distortion index DELTA, a 2 theta -range at higher angles was monitored. Compared to a theoretically calculated continuous change in cell parameters during the transformation, the data obtained after annealing for various lengths of time clearly show a discontinuous development with well Al,Si-ordered orthorhombic cordierites forming in small amounts after only 4 hr and increasing in mass with continued heating at the expense of hexagonal cordierite. This metastable coexistence of two cordierite polymorphs is caused by slow reaction kinetics in the intimately intergrown polycrystalline aggregates formed by devitrification and contrasts with the abrupt transition observed in single crystals under identical heating conditions. For K-doped cordierite aggregates, the kinetically induced phase coexistence is conserved for longer heating periods the higher the amounts of K incorporated, and cordierites with intermediate cell parameters are observed periodically. On the basis of these results, earlier interpretations of NMR and IR spectra and of calorimetric data thought to have been obtained on single-phase cordierites must be reconsidered. For truly single- phase orthorhombic synthetic or natural cordierites, DELTA remains an important rapid indicator of cell parameters which are in turn dependent on the chemical composition.