Parahilgardite, Ca6[B5O9]3Cl3 · 3H2O from Iberville Parish, Louisiana is triclinic, mildly piezoelectric, space group P1, with unit cell dimensions: a = 17.495(4), b = 6.487(1), c = 6.313(1)Å; α = 60.77(1), β = 79.56(1), γ = 83.96(1)°; Z = 1. The R-factor is 0.020 for 4490 reflections. The absolute configuration and hydrogen positions have been determined. The structure of parahilgardite is a three dimensional framework, whose building blocks are three crystallographically distinct pentaborate polyanions [B5O12]9−, each consisting of three (BO4) tetrahedra and two (BO3) triangles. These polyanions, two of which are in l and one in d configuration, form three distinct single chains by sharing tetrahedral comers along the c axis. By sharing tetrahedral and triangular corners, each chain is connected to four adjacent chains, resulting in an open framework with channels parallel to all three axes, where calcium and chlorine atoms and the water molecules are located. The calcium polyhedra are hexagonal bipyramids, which are connected to form an open sheet. The water molecules are hydrogen bonded with chlorine atoms in quasi-linear chains parallel to the c axis.

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