Mullitization of halloysite was examined with special attention to the influence of polytypism and impurities on the phase transformation. The phase transformation sequence in halloysite is identical with those in kaolinite and in dickite. Difference in the phase transformation above $1000^{circ}C$ in kaolin minerals is attributed to the different crystal structures of metakaolins or the effect of impurities. Growth of mullite crytals in the discordered kaolinte including halloysite or illite occurs at about $100^{circ}C$ lower temperature. Cristobalite is also hardly produced.