TY - JOUR A1 - Freyer, D. A1 - Reck, Günter A1 - Bremer, M. A1 - Voigt, W. T1 - Thermal behaviour and crystal structure of sodium-containing hemihydrates of calcium sulfate N2 - A metastable sodium-containing hemihydrate ((6-x)CaSO₄∙xNa₂SO₄∙3H₂O, 0≤x≤1) limited by the pentasalt composition Na₂SO₄∙5CaSO₄∙3H₂O occurs as an intermediate solid phase in the systems Na₂SO₄-CaSO₄-H₂O, NaCl-Na₂SO₄-CaSO₄-H₂O, and NaCl-CaSO₄-H₂O. X-Ray structure determination of a crystal with pentasalt composition results in a super-structure of the pure hemihydrate (CaSO₄∙0.5H₂O), in which 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 forming 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 hydrate crystallized in a monoclinic space group C121 (No. 5) with a=24.1781(11), b=13.805(2), c=12.7074(12) Å, β=90.089(12)°. The dehydration 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 by the crystal sizes. The thermograms of small crystals as opposed to large ones show a exothermic effect adjoining the endothermic dehydration. This may be indicative for a change in the dehydration mechanism upon crystal size. KW - Bassanite KW - Crystal structure KW - Sodium pentasalt KW - Calcium sulfate hemihydrate KW - Thermal analysis PY - 1999 DO - https://doi.org/10.1007/pl00010180 SN - 1434-4475 SN - 0026-9247 VL - 130 SP - 1179 EP - 1193 PB - Springer CY - Wien AN - OPUS4-61831 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -