The crystal structure of CaNa2(CO3)2·5H2O (synthetic gaylussite) has been determined from 2988 single-crystal X-ray diffraction data collected by a counter method. The unit cell parameters are a = 14.361 ± 0.002 Å, b = 7.781 ± 0.001 Å, c = 11.209 ± 0.002 Å, and β = 127.84 ± 0.01°, and the space group is C2/c. There are four formula weights per cell. The calculated density is 1.99 g/ml; the observed density is also 1.99 g/ml. [formula omitted]; R = 0.054. The hydrogen atoms have been located approximately. Two CO3 anions, at a dihedral angle of 134.3° to one another, are ionically bonded edgewise to a Ca ion to form a Ca(CO3)2 triplet. The bonded edges of the CO3 groups are very nearly parallel. Both CO3 groups in a triplet are further bonded to a pair of Na ions which are also bonded to adjacent triplets. Four of the water molecules are bonded to Ca and form hydrogen bonds to CO3 groups in neighboring triplets. The fifth water molecule is bonded to two Na ions and is hydrogen bonded to two water molecules. One hydrogen appears to be only 2.2-2.5 Å from the Na. The crystal structure of CaNa2(CO3)2·2H2O (synthetic pirssonite) has also been determined from 1141 single-crystal X-ray diffraction data collected by a counter method. The unit cell parameters are a = 11.340 ± 0.004 Å, b = 20.096 ± 0.005 Å, and c = 6.034 ± 0.002 Å, and the space group is Fdd2. There are eight formula weights per cell. The calculated density is 2.35 g/ml; the observed density is also 2.35 g/ml; Rw = 0.029; R = 0.044. The hydrogen atoms have been located approximately. As in CaNa2(CO3)2·5H2O, two CO3 anions are bonded edgewise to a Ca ion to form a triplet. In this triplet, the Ca, the two carbons, and the CO3 oxygens remote from Ca lie nearly on a line, but the CO3 groups have twisted about this axis so that the bonded edges are nearly at right angles to one another; the dihedral angle is 95.5°. In contrast to CaNa2(CO3)2·5H2O, each CO3 anion is also bonded to a second Ca ion to form a threedimensional network. The Na ions and water molecules form an alternating chain along [Oil]. Each Na ion is coordinated to four oxygens in CO3 anions and to two water molecules. The water molecules are primarily coordinated to Ca ions and form hydrogen bonds with neighboring CO3 anions. The HaO…Na bonding is thought to be weak. © 1969, American Chemical Society. All rights reserved.
