Manganese phosphate hydrate crystals were prepared from aqueous solutions containing MnCl 2 , (NH 4 ) 2 HPO 4 , and citric acid. Switzerite [Mn 3 (PO 4 ) 2 •7H 2 O] and niahite (NH 4 MnPO 4 •H 2 O) phases were produced at room temperature, and then, hureaulite [Mn 5 (PO 3 OH) 2 (PO 4 ) 2 •4H 2 O] 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 Mn 2+ 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 singlecrystal 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.
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