Temperature-induced transformation of metavariscite to berlinite

The phase transition of metavariscite into berlinite has been studied by means of different techniques, namely electron probe micro analysis, environmental scanning electron microscopy, high-temperature X-ray diffraction, differential thermal analysis, thermogravimetry, thermoluminescence, and thermo Raman. The application of these techniques indicates that: (i) from room temperature to 150°C metavariscite (monoclinic dimorph AlPO₄ ? 2H₂O) appears as the main phase, (ii) in the range of 100–300°C metavariscite starts to lose the water molecules giving rise to form α-berlinite (trigonal AlPO₄) that is the stable phase up to ca 540°C, and (iii) from 550°C onwards the structure adopts the more stable configuration, tetragonal β-berlinite.