Abstract
The title carbonate hydrate, Cs(2)Mg(4)(CO(3))(5)·10H(2)O, was crystallized at room temperature out of aqueous solutions containing caesium bicarbonate and magnesium nitrate. Its monoclinic crystal structure (P2(1)/n) consists of double chains of composition (1) (∞)[Mg(H(2)O)(2/1)(CO(3))(3/3)], isolated [Mg(H(2)O)(CO(3))(2)](2-) units, two crystallographically distinct Cs(+) ions and a free water mol-ecule. The crystal under investigation was twinned by reticular pseudomerohedry.