A new representative of pentaborates with the composition Ca[B₅O₈(OH)] · H₂O was synthesized under hydrothermal conditions within the framework of the study of phase formation in the CaCl₂-Na₂CO₃-B₂O₃ system. The crystal structure of the new pentaborate was established (a = 6.5303(9) Å, b = 19.613(3) Å, c = 6.5303(9) Å, β = 119.207(2)°, V = 2513(2) ų, sp. gr. P2₁/c, Z = 4, d _calcd = 2.74 g/cm³, automated Brucker SMART CCD diffractometer, 6871 reflections, λMo radiation, anisotropic refinement by least-squares, R _hkl = 0.076). The structure of calcium pentaborate is built by isolated B-Ca-B stacks parallel to the (010) plane. The central fragments of these stacks consists of nine-vertex Ca polyhedra. The Ca layers are located between loose B-O networks composed of [B ₂ ^t B ₃ ^Δ O₈(OH)]²⁻ pentaborate groups. The arrangement of the polyhedra around large cations in pentaborates with groups of two B tetrahedra and three B triangles was analyzed in terms of crystal chemistry. It is established that the structures of these compounds consist of large isolated polyhedra, columns, layers, and three-dimensional frameworks.