Gillespite glass-ceramics

The discovery of a new glass-ceramic based on the mineral phase gillespite (BaFeSi ₄O ₁₀) is reported. Glasses having the stoichiometric barium iron silicate composition were melted to obtain a range of iron redox states. Gillespite was found to precipitate only if the iron reduction ratio, [Fe ⁺²:total Fe], in the glass was greater than 0.5. Under this condition, surface crystallization was always observed, with some large, internal gillespite crystals forming when the reduction ratio was ≥0.7. A uniform, fine-grained glass-ceramic was achieved when Pt was added as a nucleating agent. The phase assemblage typically comprised gillespite as the major phase with sanbornite (low-BaSi ₂O ₅) as a secondary phase and minor amounts of cristobalite: the gillespite crystals forming rosettes around the Pt nuclei. The relative amount of gillespite in the glass-ceramic could be increased by increasing the iron reduction ratio. Gillespite glass-ceramics have a red color which could make them useful as Cd-free red pigments.