ORIGIN OF UNUSUAL FLUOROPHLOGOPITE MORPHOLOGY IN MICA GLASS-CERAMICS OF THE SYSTEM SIO2-AL2O3-MGO-K2O-NA2O-F-2

Extruded mica glasses were exposed to various heat treatments to conve rt them into glass-ceramics exhibiting anisotropic properties. In depe ndence on duration and temperature of the treatment, different morphol ogies of the mica crystals were observed. The TEM study proved the cry stallisation of fluorophlogopite formed in the glass system used to be a two-stage process. In coexistence with an intermediate phase (norbe rgite, Mg3F2SiO4), highly defective and magnesium depleted phlogopite plates are formed in the first step. Upon dissolution of norbergite, e pitaxial growth of non-defective and nearly stoichiometric fluorophlog opite along the c-direction occurs on both sides of the defective sla b in the second step. The morphology of the resulting sandwich structu re is sensitively dependent on the tempering conditions. The origin of this effect is discussed based on the analysis of the defects observe d in the inner part of the sandwich. (C) 1998 Elsevier Science B.V. Al l rights reserved.