Triclinic ulexite crystals contain isolated borate polyanions [B 5O6(OH)6]3- related to the well known pentaborate polyanion [B5O6(OH)4] - by addition of two hydroxyl groups to two opposite B-O triangles. The isolated ulexite polyanions form the [B5O7(OH) 4]n3n- chains previously found in crystals of the related mineral probertite, NaCa-B5O7(OH) 4??3H2O.
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Pentaborate polyanion in the crystal structure of ulexite, NaCaB 5O6(OH)6??5H2O