The microstructure of vitrified kaolin ceramic tapes has been studied via scanning and transmission electron microscopy (SEM and TEM). The sintered samples contained crystalline phase of predominantly stoichiometric mullite (3Al(2)O(3).2SiO(2)), which consisted of high aspect ratio, acicular crystals that are often referred to as secondary mullite. These crystals were interlocked and embedded in an aluminosilicate glass matrix of inhomogeneous composition. The glass matrix contained an average of similar to3.63 wt% K as determined by energy-dispersive X-ray analysis (EDS), whose composition could be approximated to 5Al(2)O(3).16SiO(2).0.1MgO.0.3K(2)O.0.15TiO(2).0.12Fe(2)O(3). The acicular crystals have approximately the stoichiometric composition of Al2O3:SiO2 = 3:2. They have grown along a specific crystallographic orientation along the [001] axis. The crystal growth front exhibited facetting on the {110) planes with microfacetting on both the {100) and {010) planes.