Mica glass ceramics were produced through controlled crystallisation of a glass in the SiO2-Al2O3 -CaO-MgO-K2O-MgF system. TiO2, P2 O5 and ZrO2 were added individually as nucleating agents in the amount of 0.5 or 1.0 wt% of the glass. Crystalline phases were identified by X-ray diffraction. The effects of small amounts of various nucleating agents on the microstructure were determined by scanning electron microscopy. Microstructural analyses were supplemented with measurements of the Vickers microhardness and indentation fracture toughness. The amount and kind of nucleating agent did not change the number of crystalline phases present but had an influence on the shape and size of the crystals, and hence on the indentation fracture toughness of the mica glass ceramics. The values for the indentation fracture toughness for the mica glass ceramics containing 0.5 wt% of TiO2, P2O5 and ZrO2 were 1.69±0.09 MPa.m1/2, 1.74 ± 0.10 MPa.m1/2 and 1.77 ± 0.12 MPa.m1/2, respectively. Copyright © 2008 Inderscience Publishers.