Single crystals of Li-3[B8O12(OH)(3)] were synthesized under hydrothermal conditions using a complex initial composition that included SiO2. The crystal structure is acentric, space group P (6) over bar 2c, with a = 8.9301(2) angstrom and c = 15.9962(4) angstrom, and Z = 4. New double three-membered rings of hexaborate blocks and monoborate triangles are connected into a polyborate anionic layer. Although the new structure is similar to double ring silicate Na3Y[Si6O15], detailed comparison of the anionic groups demonstrates principal differences between silicates and borates. Sodium silicate was proposed as an intrinsic fast ion conductor, and for the new borate Li ionic conductivity is very probable, as Li atoms in it are in positions similar to the Na atoms in the silicate.