Thermal behaviour and crystal structure of sodium-containing hemihydrates of calcium sulfate

A metastable sodium-containing hemihydrate ((6-x)CaSO4. xNa(2)SO(4). 3H(2)O , 0 less than or equal to x less than or equal to 1) limited by the pentasa lt composition Na2SO4. 5CaSO(4). 3H(2)O occurs as an intermediate solid pha se in the systems Na2SO4-CaSO4-H2O, NaCl-Na2SO4-CaSO4-H2O, and NaCl-CaSO4-H 2O. X-Ray structure determination of a crystal with pentasalt composition r esults in a super-structure of the pure hemihydrate (CaSO4. 0.5H(2)O), in w hich one Ca2+ ion is statistically replaced by two Na+ ions. One Na+ cation is situated in a Ca2+ position in only one of the three chains of CaSO4 fo rming an axis of nearly three fold symmetry along the c-axis. The second Na + is located in the water channel neighbouring to the first Na+. The hydrat e crystallized in a monoclinic space group C121 (No. 5) with a = 24.1781(11 ), b = 13.805(2), c = 12.7074(12) Angstrom, beta = 90.089(12)degrees. The d ehydration temperature of the hydrates depends on their Na+ ion content. A high Na+ content (in water channels) blocks the water escape strongly, and the dehydration temperature increases. Thermal behaviour is also effected b y the crystal sizes. The thermograms of small crystals as opposed to large ones show a exothermic effect adjoining the endothermic dehydration. This m ay be indicative for a change in the dehydration mechanism upon crystal siz e.