A monoclinic (2M) polytype of sideronatrite, Na2Fe(SO4)2(OH)(H2O)3, was found at Xitieshan, Qinghai Province, China. The crystal structure was solved by direct methods and refined by full-matrix least-squares (R1 = 3.88 %) in the space group P121/n1 with a = 7.1559(14), b = 7.2845(15), c = 20.889(4) Å, β = 99.37(3)∘, V = 1074.4(4) Å3, and Z = 4, using 2207 observed reflections with I > 2σ(I). The atomic arrangement of sideronatrite-2M confirms a theoretical model in space group P21/a, previously proposed in literature.
The structure of sideronatrite-2M is based on ferric-sulfate chains of 7 Å periodicity, similar to those observed in the structures of the orthorhombic polytype and of metasideronatrite. The system of hydrogen bonds has been determined.