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Issue 8, 2010
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A new calcium sulfate hemi-hydrate

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Calcium sulfate hydrates receive significant attention due to numerous large scale industrial applications. There has been a long debate on the possible existence of two gypsum hemi-hydrate polymorphs, denoted α- and β-CaSO4·0.5H2O. In this work, a new crystal structure of calcium sulfate hemi-hydrates is presented, denoted β-CaSO4·0.5H2O. The structure was solved using powder neutron diffraction data, the space group is P31 and the unit cell in a hexagonal setting a = 6.9268(1), c = 12.7565(3) Å. The structure has two calciumoxygen coordination polyhedra: Ca1 is eight coordinated and has Ca–O bond lengths in the range 2.31(3) to 2.89(2) Å and Ca2 is nine coordinated and has one Ca–Owater bond length of 2.43(3) Å, and eight Ca–O bonds in the range 2.30(4) to 2.86(4) Å. Two sulfate ions have S–O bonds in the range 1.47(3) to 1.49(4) Å, and 1.47(3) to 1.50(3) Å, respectively. The water molecule forms a hydrogen bond of 2.55(4) Å to an oxygen atom in one of the sulfate ions. The structure of the hemi-hydrate β-CaSO4·0.5H2O has one-dimensional channels running parallel to the c-axis where the water molecules are located. This relates the structures of α- and β-CaSO4·0.5H2O and soluble anhydrite AIII-CaSO4, which all have similar channel structures. The water molecules in the structure of β-CaSO4·0.5H2O are packed in the channels with a three fold (31) symmetry in a different way as compared to the pseudo hexagonal found in the structure of α-CaSO4·0.5H2O.

Graphical abstract: A new calcium sulfate hemi-hydrate

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Article information

09 Jul 2009
23 Nov 2009
First published
13 Jan 2010

Dalton Trans., 2010,39, 2044-2048
Article type

A new calcium sulfate hemi-hydrate

A. N. Christensen, T. R. Jensen and A. Nonat, Dalton Trans., 2010, 39, 2044
DOI: 10.1039/B913648G

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