The crystal structure of tunellite, SrO??3B2O 3??4H2O, with infinite sheets of composition n[B6O9(OH)2]2-, has cations and water molecules in the spaces within the sheets. Adjacent sheets are held together by hydrogen bonding through the water molecules. The boron-oxygen polyanions provide the first example in hydrated borate crystals of one oxygen linked to three borons.
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Boron-oxygen polyanion in the crystal structure of tunellite