Fluorphlopgopite ceramic via sintering of glass using inexpensive natural raw materials

Fluorphlopgopite ceramic was prepared from kaolin, feldspar and steatite with the addition of pure chemicals Na2CO3, AlOOH·H2O and MgF2. Glass-ceramics containing the crystal phase fluorphlogopite were prepared using two different routes. The first route was to melt a glass and to crystallize fluorphlogopite by subsequent annealing at temperatures in the range from 900 to 1050 °C kept for 2 h. Here, fluorphlogopite and cordierite were formed. In the other route a sintering procedure was applied using the pre-mentioned materials and sintering temperatures in the range from 900 to 1200 °C kept for 2 h. Here, fluorphlogopite [(Na,K)Mg3(AlSi3O10)F2], spinel and norbergite were precipitated. The unit cell parameters of the formed fluorphlogopite phase are larger than those according to JCPDS data. This is caused by a partial replacement of Mg by Ca or Ba from the raw materials. The microstructure of sintered samples show interlocked fluorphlogopite rods and small spinel crystals. The samples prepared by thermal annealing of the glass showed curved fluorphlogopite and additionally tiny cordierite crystals.