Quintinite-1M, [Mg4Al2(OH)12](CO3)(H2O)3, is the first monoclinic representative of both synthetic and natural layered double hydroxides (LDHs) based on octahedrally coordinated di- and trivalent metal cations. It occurs in hydrothermal veins in the Kovdor alkaline massif, Kola peninsula, Russia. The structure was solved by direct methods and refined to R1 = 0.031 on the basis of 304 unique reflections. It is monoclinic, space group C2/m, a = 5.266(2), b = 9.114(2), c = 7.766(3) Å, β = 103.17(3)°, V = 362.9(2) Å3. The diffraction pattern of quintinite-1M contains sharp reflections corresponding to the layer stacking sequence characteristic of the 3R rhombohedral polytype, and rows of weak superlattice reflections superimposed upon a background of streaks of modulated diffuse intensity parallel to c*. These superlattice reflections indicate the formation of a 2-D superstructure due to Mg-Al ordering. The structure consists of ordered metal hydroxide layers and a disordered interlayer. As the unit cell contains exactly one layer, the polytype nomenclature dictates that the mineral be called quintinite-1M. The complete layer stacking sequence can be described as...=Ac1B=Ba1C=Cb1A=... Quintinite-1M is isostructural with the monoclinic polytype of [Li2Al4(OH)12](CO3)(H2O)3.