The aluminum magnesium spinel was obtained from Mg（OH）2 and γ-Al2O3．Synthesis and sinterability of speinel doped with AlF3 and MgF2 have been studied by X-ray powder diffractometry and TEM observation．The fluorides were hydrolyzed by water from the decomposition of Mg（OH）2．HF gas was generated，which was absorbed on the surface of MgO and Al2O3．Addition of fluoride accelerated the synthesis of spinel above 850℃．The mechanism is suggested as follows． Departure of HF from the surface of MgO and A12O3 increases the surface energy．Then MgO evaporates and deposits instantly on the surface of Al2O3 and reacts．The synthesis of spinel proceeds rapidly．
The relative density of sintered spinel whose powder was doped with fluoride and calcined at 1000℃ was hlgher than that of no-dopant which was fired above 1400℃． At 1500℃ for 2 hours the relative density of the sintered spinel with AlF3 addition and MgF2 addition was 99％ and 96％，respectively． The pores of sintered spinel with MgF2 addition whose sizes were about 2 μm were observed in the flrlng bodies．This suggests traces of decomposed MgF2 because the sizes of pores were similar to those of added MgF2．
