The crystal structure of ginorite, Ca2[B14O20(OH)6]·5H2O, and the analysis of dimensional reduction and structural complexity in the CaO–B2O3–H2O system

Abstract

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, P2₁/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 BO₄ tetrahedra and BO₃ triangles. The fundamental building block (FBB) for the crystal structures of ginorite and strontioginorite consists of eight BO₃ triangles and six BO₄ tetrahedra. The adjacent FBBs are linked by sharing O atoms to form layers that possess open nine-membered rings centered by the M1 and M2 metal atoms. In the crystal structure of strontioginorite, the M1 and M2 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 M1O₈ polyhedron 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 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.