Substitution of boron for silicon in K-fluor richterite glass ceramics

Substitution of B³⁺ for Si⁴⁺ in the K-fluor-richterite KNaCaMg₅Si₈O₂₂F₂ structure is presented through study of the crystallisation behaviour of glasses containing up to 36.55 wt% B₂O₃(B/Si=4.00 at%) upon different heat treatment regimes. Gradual additions of boron lower the melting temperatures and strongly affect nucleation and crystallisation temperatures. The effect of boron replacement on the type of crystallising phases was manifested by DTA, XRD and EDAX. A single K-fluor-richterite phase resulted in crystallisation of glasses with 17.34 wt% B₂O₃(B/Si=0.80 at%). For greater boron contents (>4B per formula unit), borate mineral phase crystallised, namely suanite Mg₂B₂O₅ in addition to fluorite CaF₂ and fluorphlogopite KMg_2.5Si₄1O ₀F₂. In the case of maximum substitution (8B per formula unit, i.e. B/Si=4.00 at%) suanite becomes the main phase, K-fluor-richterite disappears, with fluorphlogopite in addition to fluorite. Incorporation of boron in the crystalline K-fluor-richterite changes the unit cell parameters. In comparison with the pre-reported unit cell constant of B free K-fluor-richterite relative decrease in the a, b and c axes with increase the β angle are detected in B containing K-fluor-richterite. A gradual increase in crystal growth rate takes place in the monoclinic K-fluor-richterite phase with increasing boron content. The boron rich sample shows microstructures of interlocked fluormica flakes.