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, 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].
Highlights
The natural evaporitic mineral carnallite, KClÁMgCl2Á6H2O, is a main source for potash fertilizer production and for the production of magnesium chloride
Crystallization experiments with natural and synthetic potash salt brines revealed the formation of a new phase of potassium carnallite, i.e. KClÁMgCl2Á6H2O
Crystal structure analysis yielded a monoclinic form of potassium carnallite, for which the crystal structure was solved
Summary
The natural evaporitic mineral carnallite, KClÁMgCl2Á6H2O, is a main source for potash fertilizer production and for the production of magnesium chloride. The structures of carnallites are typically described as perovskite-like, with the halide anions arranged octahedrally around the monovalent cations. The corners of these octahedra are linked to neighbouring octahedra, forming a cubic lattice with large holes having cuboctahedron geometry. An alternative description of the perovskite lattice is a cubic close-packed stacking of anions with cations (here M) occupying the octahedral holes. This is valid for all known carnallite-type structures, except for the mineral carnallite itself, where every third layer consists of a hexagonal stack denoted as hcc. Selection of a single crystal and performance of a preliminary lattice determination suggested the presence of a new phase
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