Crystallization of metastable monoclinic carnallite,KCl·MgCl2·6H2O: missing structural link in the carnallite family

During evaporation of natural and synthetic K–Mg–Cl brines, the formation of almost square plate‐like crystals of potassium carnallite (potassium chloride magnesium dichloride hexahydrate) was observed. A single‐crystal structure analysis revealed a monoclinic cell a = 9.251 (2), b = 9.516 (2), c = 13.217 (4) Å, β = 90.06 (2)° and space group C2/c]. The structure is isomorphous with other carnallite‐type compounds, such as NH₄Cl·MgCl₂·6H₂O. Until now, natural and synthetic carnallite, KCl·MgCl₂·6H₂O, was only known in its orthorhombic form a = 16.0780 (3), b = 22.3850 (5), c = 9.5422 (2) Å and space group Pnna].