The hydrated magnesium borate mineral hungchaoite, MgO.2B2O3.9H20, from Furnace Creek borate area. Death Valley, California, is triclinic, space group PI, with cell dimensions: a = 8.807(1), b = 10.657(1), с = 7.897(l)A, α = 103.39(1), β = 108.53(1), γ = 97.18(1)°, and Z = 2. The crystal structure has been determined by the symbolic addition method and difference Fourier syntheses, and refined by the method of least squares to an Л-factor of 0.049, based on 4860 reflections measured on an automatic single-crystal X-ray diffractometer. All hydrogens have been located and their positions refined.

The crystal structure consists of a molecular complex, [Mg(H2O)5B4O5(OH)4], and two water molecules, hydrogen-bonded into a three-dimensional framework; the molecular complex consists of a [Mg(H2O)5(OH)] octahedron and the tetraborate polyanion [B4O5(OH)4]2− (4:2Δ+2T) sharing a common tetrahedral (OH) corner. The orientation of the Mg-octahedron with respect to the tetraborate polyanion is fixed by means of an intramolecular hydrogen bond between a water molecule bonded to magnesium and an oxygen atom belonging to the tetraborate polyanion. The average Mg-O, B-O(tetrahedral), and B-O (triangular) bond lengths are: 2.084, 1.476, and 1.368A respectively. All hydrogen atoms are involved in hydrogen bonding. The average O-H, H … O, and O-H … О distances are 0.86, 1.94, and 2.781 A respectively. All the hydrogen bonds are nearly straight (av. O-H … О angle 171°), except one, which is bent (O-H …O angle 111°). The two isolated water molecules show a distorted tetrahedral coordination, whereas those bonded to magnesium show a distorted trigonal planar coordination

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