A complete crystalline solid solution exists between magnesium cordierite (Mg2Al4Si5O18) and nickel cordierite (Ni2Al4Si5O18). Isothermal crystallization experiments were conducted at 1 atm on stoichiometric powdered glasses in this compositional range in which high quartz crystallized and then transformed to hexagonal cordierite. When nickel is present, these three phases can be simply identified by colour changes : the glass is brown, the high quartz is red and the cordierite is yellow. The sequence of appearance of the phases is independent of the nickel content of the glass. At any given temperature, the higher the nickel content, the shorter are both the time required for crystallization of high quartz from the glass and that required for the transformation to hexagonal cordierite. This implies that the processes involved in both steps might be similar. Measurements of lattice constants of cordierite crystallization products show a linear decrease in a and a linear increase in c as a function of increasing nickel content between the magnesium and nickel end members. There is no change of the cell volume with the substitution of nickel into the structure. The increase in reaction rates caused by increasing the Ni/Mg ratio of the glass starting material may be due to the stronger network modifying effect of nickel in comparison to magnesium.