Crystal structure and characterization of magnesium carbonate chloride heptahydrate

MgCO$_{3}$·MgCl$_{2}$·7H$_{2}$O is the only known neutral magnesium carbonate con­taining chloride ions at ambient conditions. According to the literature, only small and twinned crystals of this double salt could be synthesised in a concentrated solution of MgCl$_{2}$. For the crystal structure solution, single-crystal diffraction was carried out at a synchrotron radiation source. The monoclinic crystal structure (space group Cc) exhibits double chains of MgO octa­hedra linked by corners, connected by carbonate units and water mol­ecules. The chloride ions are positioned between these double chains parallel to the (100) plane. Dry MgCO$_{3}$·MgCl$_{2}$·7H$_{2}$O decom­poses in the air to chlorartinite, Mg$_{2}$(OH)Cl(CO$_{3}$)·nH$_{2}$O (n = 2 or 3). This work includes an extensive char­acterization of the title com­pound by powder X-ray diffraction, thermal analysis, SEM and vibrational spectroscopy.