HYDROGEN-BONDING IN COLEMANITE - AN X-RAY AND STRUCTURE-ENERGY STUDY

The crystal structure of colemanite, CaB3O4(OH)3.H2O, monoclinic, a 8.712(2), b 11.247(3), c 6.091(1) angstrom, beta 110.12(2)degrees, V 560.4(2) angstrom3, Z = 4, space group P2(1)/a, has been refined by full-matrix least-squares to an R index of 2.8% and a wR index of 3.5% for 1408 unique observed [\F(O)\ greater-than-or-equal-to 5sigma(F(O))] reflections measured with MoKalpha X radiation. The H-positions were located on difference-Fourier maps and refined using the ''soft'' constraints that H-O distances are approximately 0.97 angstrom. Colemanite is a complex inoborate. Each chain element consists of two Bphi4 (phi: unspecified anion) tetrahedra and a Bphi3 triangle linked through corners to form rings of composition [B3O5(OH)3]4-; these rings link via comer-sharing to form chains of composition [B3O4(OH)3]2- that extend along [100]. The eight-coordinate Ca polyhedra share corners to form chains parallel to [100]. These link with the B polyhedral chains via comer sharing to form complex heteropolyhedral sheets parallel to (010). The linkage between these sheets is weak, consisting of a small number of B-phi-Ca linkages together with a network of hydrogen bonds, and accounts for the perfect (010) cleavage in colemanite. Minimum-energy calculations of the hydrogen positions done with a directionally isotropic H-O potential function predict the positions of three of the five hydrogen atoms, but fail to give the correct positions for two hydrogen atoms. This is attributed to the lack of a directional component in the potential function.