Hilgardite, Ca2B5O9Cl · H20, from Choctow Salt Dome, Iberville Parish, Louisiana, is monoclinic, space group Aa, with unit-cell dimensions a = 11.438(2), b = 11.318(2), c = 6.318(1)A, β = 90.06(1)°, and Z = 4. The crystal structure has been determined from threedimensional Patterson and Fourier syntheses and refined by full-matrix least-square methods to an R-factor of 0.017 based on 1487 reflections, measured on an automatic single-crystal X-ray diffractometer. The absolute configuration and the hydrogen positions have been determined.

The structure of hilgardite is an open three-dimensional borate framework, whose building block is the anhydrous pentaborate polyanion [B5O12]9−, consisting of three (BO4) tetrahedra and two (B03) triangles. The average tetrahedral and triangular B-O distances are 1.474 and 1.363A respectively. The polyanions form three-tetrahedral-repeat single chains (6.3A c axis) by sharing tetrahedral corners with those belonging to adjacent polyanions. Within each chain, corners of two borate tetrahedra point along +a and +b directions, whereas corners of two borate triangles point along −a and −b directions; these corners are shared with four adjacent chains, such that tetrahedral corners of one chain are shared with triangular corners of the other. The resulting framework has ∼6 and ∼5A diameter open channels parallel to the a and c axes respectively. The water molecules and the chlorine atoms within the channels form quasi-linear hydrogen-bonded chains parallel to the c axis. Hilgardite may be the precursor of a new family of borate zeolites.

The calcium atoms occur within channels parallel to the a and b axes. The [Ca(l)05Cl(H20)] and [Ca(2)O6CI2] coordination polyhedra are slightly distorted pentagonal and hexagonal bipyramids, with average Ca-O distances 2.465 and 2.535A, and average Ca-C1 distances 2.817 and 2.905A respectively. By sharing opposite Cl corners the Ca(2) polyhedra form linear chains parallel to the a axis, crosslinked into a sheet by the Ca(l) polyhedra sharing polyhedral corners and edges.

Hilgardite is mildly piezoelectric, the strongest electric axis being parallel to the a axis. The piezoelectricity along b is zero or nearly zero, because the borate triangles point alternately along +b and −b, whereas along the a axis, all borate triangles point along −a. All borate tetrahedra point along −c, the c axis presumably being the intermediate piezoelectric axis.

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