Three Crystalline Polymorphs of KFeSi04, Potassium Ferrisilicate

Orthorhombic α-KFeSi0₄ ( a =0.5478, b =0.9192, c =0.8580 nm), hexagonal β-KFeSiO₄ (a =0.5309, c =0.8873 nm), and hexagonal γ-KFeSi0₄ (a =0.5319, c =0.8815 nm) were synthesized by devitrification of KFeSiO₄ glass. Powder X-ray diffraction data are given for all three polymorphs. Alpha KFeSiO₄, the high-temperature polymorph, melts congruently at 1197”±2”C. Mössbauer spectroscopy of the α phase indicates that Fe³⁺ occupies two tetrahedral sites in the lattice. Beta KFeSiO₄, the low-temperature polymorph, and γ-KFeSiO₄, a metastable polymorph, appear to be isomorphous with kalsilite, KAISiO₄, and synthetic kaliophilite, KAISiO₄, respectively, and it is proposed that β- and λ-KFeSiO₄ are linked by Si-Fe order-disorder. Beta KFeSiO₄ transforms slowly into α-KFeSi0₄ above 910°C but the transformation was not shown to be reversible in the present dry-heating experiments.