Crystal structure of a new polymorphic modification of niahite, NH4MnPO4 · H2O

The crystal structure of a new modification of the niahite mineral NH4MnPO4 · H2O, obtained in the form of single crystals under hydrothermal conditions in the Mn2O3-(NH4)3PO4-H2O system, is determined by X-ray diffraction (Xcalibur-S-CCD diffractometer, R = 0.0259): a = 17.582, b = 4.909, and c = 5.731 Å; space group D2h16 = Pnam, Z = 4, Dx = 2.497 g/cm3. It is shown that the new compound is a polymorphic modification of niahite; it has a layered structure, which differs from the structure of the mineral by the presence of the center of inversion and the doubled a unit cell parameter. It is proposed to consider the minerals and synthetic analogues of the niahite and triphylite groups as members of the common polysomatic series.