Crystal structure of a new synthetic calcium pentaborate, Ca2[B5O8(OH)]2 · [B(OH)3] · H2O, and modular crystal chemistry of pentaborates with polar boron-oxygen layers

The crystal structure of a new calcium pentaborate, Ca2[B5O8(OH)]2 B(OH)3 · H2O, synthesized by the hydrothermal method, has been determined (a = 6.6620(4) Å, b = 6.5990(4) Å, c = 10.5830(6) Å, α = 103.372(5)°, β = 78.296(5)°, γ = 120.931(7)°, space group P1, Z = 1; R1 = 0.0712 for 4263 reflections with I > 2σ (I)). This structure is built up of three-layer stacks isolated from each other and consisting of Ca (Sr, Ba) polyhedra joined into columns with boron-oxygen networks located on both sides. Building blocks of the network are the pentaborate groups [B2tB3ΔO8(OH)]2− formed by two boron tetrahedra and three boron triangles. A comparative crystal chemical analysis has been performed for divalent metal pentaborates that have similar compositions and structures. A common fundamental building block {M[B5O8(OH)]}2 · {[B(OH)3],H2O}2 (M = Ca, Sr, Ba) is determined. The diversity of mineral species in the subgroup of pentaborates under consideration is associated both with the variations in the composition and symmetry of individual blocks and with the types of their joining in the structure.