The phase change of a natural hemimorphite sample from Minas Gerais (Brazil) was investigated by two X-ray diffraction (XRD) methods and by near-infrared reflectance spectroscopy. Applying successive thermal treatments, the crystal structure undergoes two orientation conversions. The first one occurs at about 550°C, and it was revealed by the Laue method. Below 500°C, the water molecules were partially expelled without changing the crystal structure. A fact that supports this statement is the sequential disappearance of the water bands at 1400 and 1900nm by thermal treatment. The second conversion takes place below 939°C. Moreover, at 972°C a phase change to the willemite mineral (-Zn2SiO4) has been observed. This last conversion was confirmed by the power XRD. In addition, natural hemimorphite displayed a high pyroelectricity, which is related both to the absence of inverse centre and to the presence of molecular water and hydroxyl groups in the crystal structure.