Dumortierite, basic aluminum borosilicate, is a rare mineral in Japan which was found by the writer from a pyrophyllite deposit of the Tsukigata mine, Fgkushima Prefectgre. The optical properties, electron microscope observation, infrared spectrum and X-ray powder diffraction data of the mineral show that the mineral is dumortierite. This mineral is converted into mullite on heating at 1300°C for one hour.