The crystal structure of ginorite, Ca₂[B₁₄O₂₀(OH)₆]·5H₂O, from the Chelkar salt dome, western Kazakhstan, has been refined at 150(2) and 296(2) K. The mineral is monoclinic, P 2₁/_c, unit-cell parameters are (at 150/296 K): a = 12.738(1)/12.728(1), b = 14.240(1)/14.303(1), c = 12.750(1)/ 12.755(1)Å, β = 101.163(2)/101.147(2)°, V = 2268.9(4)/2278.3(4)ų, Z = 4. The crystal structure is isotypic to that of strontioginorite and is based upon 2-dimensional anionic layers with the composition [B₁₄O₂₀(OH)₆]^4-. The layers are coplanar to (0 1 0) and are composed of BO₄tetrahedra and BO₃triangles. The fundamental building block (FBB) for the crystal structures of ginorite and strontioginorite consists of eight BO₃triangles and six BO₄tetrahedra. The adjacent FBBs are linked by sharing O atoms to form layers that possess open nine-membered rings centered by the M 1 and M 2 metal atoms. In the crystal structure of strontioginorite, the M 1 and M 2 sites are occupied by Sr and Ca, respectively, whereas, in ginorite, both sites are occupied by Ca. The Ca ↔ Sr substitution affects the geometry of the M1O₈polyhedron significantly, with the difference |Δ| between the Sr-O and Ca-O bond lengths up to 0.157Å. The dimensional reduction, structural and chemical complexity of 45 minerals and inorganic compounds of the CaO–B₂O₃–H₂O system have been analyzed using ternary diagrams. The structures with 0-dimensional structural units (finite clusters) constitute 53.3% (24) of all structures in the system, whereas 1-, 2- and 3-dimensional borate polyanions have been observed in 15.6% (7), 20.0% (9) and 11.1% (5) of all structures in the system, respectively. The average structural complexity for the 42 B-bearing phases in the CaO-B₂O₃–H₂O system is 353 bits/cell and agrees well with the average complexity of 340 bits/cell for all boron minerals. Ginorite belongs to the class of very complex structures (1536.000 bits/cell) and is the second in complexity in the system after alfredstelznerite, Ca₄(H₂O)₄[B₄O₄(OH)₆]₄·15H₂O (4026.707 bits/cell). In general, structural and chemical complexities correlate with each other and indicate the presence of the high-complexity areas in the H₂O dominant part of the diagram, in a band between ca. 10 and 20 mol% CaO.