The investigation of the crystalline phases development in Macor® glass-ceramic

The commercially available, machinable, fluormica glass-ceramic, Macor®, was remelted to produce a glass. This glass was then characterized by differential scanning calorimetry (DSC), combined differential thermal analysis/thermal gravimetric analysis (DTA/TGA), X-ray powder diffraction (XRD) and 19F magic angle spinning −nuclear magnetic resonance (MAS-NMR). The Macor® glass was shown to crystallize to chondrodite (Mg5F2(SiO4)2), followed by the conversion of chondrodite to norbergite (Mg3SiO4F2) and the conversion of norbegite to a potassium fluorphlogopite phase (KMg3AlSi3O10F2). The glass exhibited an optimum nucleation temperature just above its glass transition temperature, which is indicative of a nucleation route involving amorphous phase separation. The 19F MAS-NMR spectra showed the fluorine environments being present as F-Mg(3) in the original glass, the chondrodite, norbergite and fluorphlogopite phases, indicating that the fluorine structure is conserved throughout the crystallization process. The activation energies for crystallization were found to be 215, 431 and 251 kJ mol−1 for chondrodite, norbergite and fluorphlogopite, respectively.