Chaidamuite (ZnFe(SO_4)_2(OH). 4H_2O), which is a new sulphate mineral, belongs to the triclinic system, space group P1, with the cell parameters corrected by the least-squares method: a=7.309(2), b=7.202(2), c=9.691(3), α=89.64(3), β=105.89(3), γ=91.11(2)°, Z=2. The crystal structure has been determined by the Patterson method and Fourier syntheses and refined by the full-matrix least-squares method to an R factor of 0.032, using 2833 independent reflections. In the structure, a zigzag chain consists of [Fe(1)O_5(OH)] and [Fe(2)O_5(OH)] octahedra sharing the OH corners, and an octahedral-tetrahedral chain running parallel to the b axis consists of the zigzag chain of Fe octahedra and (SO_4) tetrahedra sharing four pairs of octahedral corners on either side of the zigzag chains. These chains are cross-linked by the isolated [Zn(1)O_2(H_2O)_4] and [Zn(2)O_2(H_2O)_4] octahedra into corrugated sheets parallel to the (100) plane. Adjacent sheets are hydrogenbonded through water molecules.