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

Abstract

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