Aegirine is synthesized at temperatures as low as 200degreesC in the system Si, Fe, Na, H2O. Aegirine is mixed with a poorly crystallized 2:1 ferric phyllosilicate. At lower temperature, only 2:1 phyllosilicates are synthesized. A thermodynamic approach shows that the synthesized aegirine is in equilibrium with ferripyrophyllite, and that the stability field of aegirine at lower temperature is strongly reduced by a 2:1 phyllosilicate with a tetrahedral charge due to tetrahedral substitutions of Si for Fe3+.