From glasses with the composition 21 2 MgO 21 2 Al2O3 51 9 SiO2 5 7 ZrO2 (in mol%) glass-ceramics were prepared by thermal annealing at temperatures in the range from 950 to 1200 C The prepared glass-ceramics were characterised by dilatometry X-ray diffraction and transmission electron microscopy From the XRD-patterns Rietveld refinement was done in order to study the transformation from beta- to alpha-quartz-solid-solution After annealing at temperatures <= 1000 C beta-quartz-solid-solutions were detected after cooling The beta-quartz-solid-solution is stabilized by MgO and Al2O3 With increasing annealing temperature and time crystalline MgAl2O4 was formed and the quartz-solid-solution was depleted in magnesia and alumina During cooling the solid solution is then successively transformed into the a-quartz modification This also explains the increasing thermal expansion coefficient measured by dilatometry Furthermore with increasing the annealing time the transformation temperature of the residual glassy phase increases to a value (968 degrees C) slightly above the annealing temperature of 950 C Then the crystals are embedded in a highly viscous layer which hinders further crystal growth (C) 2010 Elsevier B V All rights reserved