Synthesis of anorthite by the Pechini process and structural investigationof the hexagonal phase

Stoichiometric CaAl2Si2O8 gels prepared by the Pechini process crystallize to triclinic anorthite via an intermediate, previously unknown pseudohexago nal phase. Hydrothermal treatment of this pseudohexagonal phase at 350 degr ees C transforms it to hexagonal anorthite. These combined processes produc e the hexagonal phase via a low-temperature, chemical route in contrast to the conventional high-temperature melting and crystallization method. Powde r x-ray diffraction data and nuclear magnetic resonance (NMR) spectra of th e hexagonal anorthite agree well with the published structure. They indicat e a high degree of tetrahedral Si/Al ordering within the layers, but long-r ange disorder in the average structure due to (001) stacking faults present between the layers.