Changing lattice metrics of synthetic cordierites: The metastable hexagonal to orthorhombic transformation sequence during isothermal annealing

Abstract

Starting from glasses heated at 1290°C, the irreversible transition of initially formed hexagonal high-into stable orthorhombic low-cordierite was studied for pure Mg 2 Al 4 Si 5 O 18 as well as for potassium-doped cordierites according to the scheme KAlSi −1 with 0.10 and 0.25 K per formula unit, using the Guinier method and synchrotron X-ray powder diffraction, respectively. Instead of the classical distortion index Δ, a 2θ-range at higher angles was monitored. Compared to a theoretically calculated continuous change of lattice metrics 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 four hours 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 (Wunder et al., 1991). For K-doped cordierite aggregates, the kinetically induced phase coexistence is conserved for longer heating periods the higher the amounts of potassium incorporated, and cordierites with intermediate lattice metrics are observed periodically. On the basis of these results, previous interpretations of NMR- and IR-spectra, as well as of calorimetric data that were thought to have been obtained on single-phase cordierites must be reconsidered, because they were also made on polycrystalline, polyphase aggregates. Thus, it is not clear as to whether or not the Al,Si-ordering process in cordierite is truly continuous. For all polyphase cordierite products formed after intermediate heating times, the classical distortion index Δ is an artefact that yields at most the approximate mass ratio between the hexagonal and orthorhombic polymorphs. Nevertheless, it may still be a useful qualitative parameter indicating the stage of transition. For truly single-phase orthorhombic synthetic or natural cordierites. Δ remains an important rapid indicator of lattice metrics which in turn are largely dependent on the chemical composition