The crystal structures of soluble anhydrite, gamma-CaSO4, and the two calcium sulfate subhydrates, CaSO4 . 0.5 H2O and CaSO4 . 0.6 H2O, were investigated by powder diffraction methods; using synchrotron X ray radiation and neutron powder diffraction, and the positions of the water molecules in the structure were determined. gamma-CaSO4 is orthorhombic, space group C222 (N degrees 21) with a = 12.0777(7), b = 6.9723(4), c = 6.3040(2) a (from the neutron diffraction data), and Z = 6. The structure contains two different chains of CaO8 coordination polyhedra (mean Ca-O = 2.460 Angstrom) held together by SO42- ions. CaSO4 . 0.5 H2O (SH1) is monoclinic, space group I121 (No. 5) with a = 12.0317(4), b = 6.9269(2), c = 12.6712(3) Angstrom, beta = 90.27(1)degrees (from the neutron diffraction data), and Z = 12. The structure contains chains of alternating CaO8 and CaO9 coordination polyhedra held together by the SO42- ions. The average Ca-O distances in the CaO8 and in the CaO9 coordination polyhedra are 2.469 and 2.500 Angstrom, respectively. A monoclinic structure is proposed for CaSO4 . 0.6 H2O (SH2), space group I121 (No. 5) with a = 11.9845(4), b = 6.9292(2), c = 12.7505(3) Angstrom, beta = 90 degrees (from the neutron diffraction data), and Z = 12. The structure contains chains of CaO8 and chains of CaO9 coordination polyhedra, both held together by the SO42- ions. The average Ca-O distances in the CaO8 and in the CaO9 coordination polyhedra are 2.538 and 2.494 Angstrom, respectively. All of the three structures have channels where the water molecules associated with the CaO9 polyhedra are located in the case of subhydrates. In the case of the hemihydrate, the water molecules form hydrogen bonds with oxygen atoms of the sulfate tetrahedra of 2.85-3.03 Angstrom. In the case of CaSO4 . 0.6 H2O, two adjacent water molecules in the channels are linked by a strong hydrogen bond (O-H = 1.9 Angstrom). (C) 1995 Academic Press, Inc.
