Structures of three dehydration products of bischofite from in situ synchrotron powder diffraction data (MgCl2.nH2O; n = 1, 2, 4).

High-quality in situ synchrotron powder diffraction data have been used to investigate the decomposition products of bischofite in the temperature range 298 </= T </= 873 K. At least eight phases could be identified: MgCl(2).nH(2)O (n = 1, 2, 4 and 6), MgOHCl.nH(2)O (0 </= n </= 1.0), MgCl(2) and MgO. The crystal structures of three magnesium chloride hydrates MgCl(2).nH(2)O (n = 1, 2, 4) were determined ab initio, replacing published Rietveld refinements from low-quality powder diffraction data based on similarity criteria. MgCl(2).4H(2)O was found to be disordered and has been correctly determined for the first time. The crystal structures of bischofite and MgCl(2).4H(2)O consist of discrete Mg(H(2)O)(6) and MgCl(2)(H(2)O)(4) octahedra, respectively. The crystal structure of MgCl(2).2H(2)O is formed by single chains of edge-sharing MgCl(2)(H(2)O)(4) octahedra, while in the case of MgCl(2).H(2)O double chains of edge-sharing MgCl(H(2)O)(5) octahedra are found. The phases in the system MgCl(2)-H(2)O are intermediates in the technologically important process of MgO and subsequently Mg production. The same phases were recently found to be of key importance in the understanding of cracks in certain magnesia concrete floors.