Isotypic compounds [Mg₃(H₂O)₁₄][M(CN)₆]₂·4HMT·xH₂O (HMT = hexamethylenetetramine or urotropine) with M = Fe, Cr, and Co are crystallized from aqueous solutions. The iron compound (M = Fe) is most remarkable because it crystallizes with distinctive thin, deep-yellow crystal plates. The crystal structure of [Mg₃(H₂O)₁₄][Fe(CN)₆]₂(C₆H₁₂N₄)₄(H₂O)_4.18 is solved and refined by X-Ray single-crystal diffraction, revealing the tetragonal space group P4c2, with a = 14.8156(5) Å, and c = 32.890(3) Å (Z = 4). The structure contains finite cyanido-bridged linear sequences of the type [(H₂O)₅Mg–(Fe(CN)₆)–(Mg(H₂O)₄)–(Fe(CN)₆)–Mg(H₂O)₅] being arranged parallel to the tetragonal axis with HMT and crystal water molecules in-between these patterns. The straightforward synthesis and the characteristic appearance of crystals of this compound can make this reaction a useful test to proof the presence of Mg²⁺ ions in aqueous solution. Thermoanalytical studies, designed to investigate the loss of water molecules and the decomposition pathway, reveal this compound as a useful precursor for the preparation of ferromagnetic cementite (Fe₃C).