Searlesite, NaBSi2O5(OH)2 is monoclinic, space group P21, with cell dimensions: a = 7.9814(12), b = 6.0657(8), c = 4.9054(4)Å, and β = 93.95(1)°. The absolute configuration and hydrogen positions have been determined from a refinement of the structure using 811 reflections measured by CuKα radiation, and the atomic coordinates determined by Kravchenko (1964). The structure has been refined using anisotropic temperature factors for all atoms except hydrogen to an R factor of 0.035 for 1694 reflections measured by MoKα radiation. The crystal structure of searlesite consists of highly kinked pyroxene-type silicate chains parallel to the c axis, connected through corner-sharing BO2(OH)2 tetrahedra to form borosilicate sheets parallel to the (100) plane; these sheets are held together by spiral columns of edge-sharing Na-O octahedra running parallel to the b axis (as found by Kravchenko, 1964) and by one of the two types of hydrogen bonds. Within the tetrahedral BO2(OH)2 group, the O-H distances are 0.87 and 0.88Å and the B-O-H angles are 114° and 113°. Both hydrogen atoms are involved in hydrogen bonds, the H · · · O distances being 2.32 and 2.04Å, and the O-H · · · O angles 147° and 170°. The bridging B-O distances (av. 1.490Å) are much longer than the non-bridging B-O distances (av. 1.455Å). Within each of the two crystallographically distinct SiO4 tetrahedra, the Si-O bond involving oxygen belonging to the Si-O-B bridge is significantly shorter (av. 1.591Å) than those involving oxygens belonging to the Si-O-Si bridges (Av. Si-O 1.617Å). The average B-O, Si(1)—O and Si(2)-O distances are 1.472, 1.614, and 1.608Å respectively. The NaO2(OH)4 octahedron is highly distorted; the average Na-O distance is 2.485Å.

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