Alumina-silica fibers, produced by air blowing a melt of 47 wt % alumina and 53 wt % silica, are initially amorphous, but may be heat treated to convert them into mullite and increase crystallite size. On heating to near 1000 degrees C, an exothermic phase transition abruptly produces mullite X-ray peaks. Further 1 hour heat treatments gradually sharpen these peaks until a minimum peak width is reached near 1600 degrees C. Scherrer calculations of crystallite size show that the mullite crystallites formed near 1000 degrees C are about 13 nm in diameter; these regions grow to about 100 nm after 1 hour at 1600 degrees C.