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 strong v 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 fluor 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 KMg2.5Si4O10F2. 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 P 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.