Aqueous Synthesis of Manganese Phosphate Hydrate Crystals for Creating Inorganic Pigment Materials

Manganese phosphate hydrate crystals were prepared from aqueous solutions containing MnCl2, (NH4)2HPO4, and citric acid. Switzerite [Mn3(PO4)2·7H2O] and niahite (NH4MnPO4·H2O) phases were produced at room temperature, and then, hureaulite [Mn5(PO3OH)2(PO4)2·4H2O] particles were obtained by aging at 80 °C or by hydrothermal treatment at 100–180 °C. The lower aging temperature (80 °C) and the chelation of Mn2+ ions by citric acid provided a slower crystal growth, resulting in large hexagonal prism blocks of ca. 500 μm in size. These were found to be single crystals of hureaulite by powder and single-crystal X-ray diffraction analysis. Diffuse reflectance spectroscopy and evaluation using L*a*b* color parameters indicated that the large single crystals present a vivid pink color and high transparency.