The crystal structure of a newly synthesized compound Na_2Ba_2B_(10)O_(17)(OH)_2 has been determined (Syntex P1-bar diffractometer, MoK_α radiation, 1784 crystallographically nonequivalent reflections, anisotropic approximation, R = 1.7). The parameters of the monoclinic unit cell are a = 11.455(7), b = 6.675(4), c = 9.360(7) A, β = 93.68(5)°, Z = 2, sp. gr. C2. The structure consists of double pseudohexagonal layers built by BO_4-tetrahedra and BO_3-triangles forming three-membered rings of two mutually orthogonal orientations. The neighboring layers along the 001 direction are bound by Na-polyhedra and hydrogen bonds with participation of OH groups. The interlayer tunnels along the 100 direction are filled with columns of Ba-polyhedra. The crystallochemical characteristics of a number of synthetic Ba-borates (to which the structure of new decaborate is related) are considered in terms of borate building blocks singled out in the structure.
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