Mixed potassium manganese vanadate carbonate, K2Mn3(VO4)(2)(CO3), represents a novel structure type; it has been synthesized hydrothermally from the system MnCl2- K2CO3-V2O5-H2O. Its hexagonal crystal structure was determined by single-crystal X-ray diffraction with a = 5.201(1) angstrom, c = 22.406(3) angstrom, space group P6(3)/m, Z = 2, p(c) = 3.371 g/cm(3), and R = 0.022. The layered structure of the compound can be described as a combination of honeycomb-type modules of [MnO6] octahedra and [VO4] tetrahedra, alternating in the [001] direction with layers of [MnCO3] built by [MnO5] trigonal bipyramids and [CO3] planar triangles, sharing oxygen vertices. The k(+) ions are placed along channels of the framework, elongated in the [100], [010], and [110] directions. The title compound exhibits rich physical properties reflected in a phase transition of presumably Jahn Teller origin at T-3 = 80-100 K as well as and T-1 = 2 K into a weakly ferromagnetic ground state, as evidenced in resonance measurements. A negative Weiss temperature = 114 K and mu(2)(B) per formula unit suggest that antiferromagnetic exchange interactions in a high-spin state, S = 5/2, in the octahedral environment and a low-spin two successive magnetic phase transitions at T-2 = magnetization, specific heat, and X-band electron spin strongly reduced effective magnetic moment mu(2)(eff) similar to 70 mu(2)(B) dominate in the system. Divalent manganese is present state, S = 1/2, in the trigonal-bipyramidal coordination.