Crystal structures of sodium magnesium selenate decahydrate, Na 2 Mg(SeO 4 ) 2 ·10H 2 O, a new selenate salt, and sodium magnesium selenate dihydrate, Na 2 Mg(SeO 4 ) 2 ·2H 2 O.

Metal selenates crystallize in many instances in isomorphic structures of the corresponding sulfates. Sodium magnesium selenate decahydrate, Na2 Mg(SeO4 )2 ·10H2 O, and sodium magnesium selenate dihydrate, Na2 Mg(SeO4 )2 ·2H2 O, were synthesized by preparing solutions of Na2 SeO4 and MgSeO4 ·6H2 O with different molar ratios. The structures contain different Mg octahedra, i.e. [Mg(H2 O)6 ] octahedra in the decahydrate and [MgO4 (H2 O)2 ] octahedra in the dihydrate. The sodium polyhedra are also different, i.e. [NaO2 (H2 O)4 ] in the decahydrate and [NaO6 (H2 O)] in the dihydrate. The selenate tetrahedra are connected with the chains of Na polyhedra in the two structures. O-H...O hydrogen bonding is observed in both structures between the coordinating water molecules and selenate O atoms.