Mica schist as fluxing material in tile compositions

Abstract

Mica schist, a rock composed mainly of fine grained muscovite mica and quartz with orthoclase and microcline feldspars as accessory minerals, was used to substitute feldspars from pegmatite in tile recipes. The rock is easily crushed and ground and contains a high proportion of silica and potash (71 and 6·9% respectively). Compositions containing 40% fine powder (< 50 μm) were used. The presence of feldspar and quartz caused dissolution of the mica in the melt and inhibited a tendency towards pinholing resulting from separate melting of the mica grains. The crystallisation of mullite doubled the modulus of rupture. Addition of borate, however, reduced the content of mullite, which in turn decreased the modulus of rupture and caused separate melting of the mica.