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

Abstract

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 , = 90.06 (2) and space group C2/c]. The structure is isomorphous with other carnallite-type compounds, such as NH 4 ClÁMgCl 2 Á6H 2 O. Until now, natural and synthetic carnallite, KClÁMgCl 2 Á6H 2 O, was only known in its orthorhombic form [a = 16.0780 (3), b = 22.3850 (5), c = 9.5422 (2) Å and space group Pnna].