A revision is presented of the restrained Rietveld analysis of the crystal structure of magnesium perchlorate tetrahydrate, Mg(ClO(4))(2)·4H(2)O, recently published by Robertson & Bish [(2010), Acta Cryst. B 66, 579-584]. The actual symmetry of the material is shown to be C2/m. The corrected structure model is refined by the derivative difference method [Solovyov (2004). J. Appl. Cryst. 37, 743-749], anisotropically for all non-H atoms and isotropically for two independent H atoms without restraints.
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