Hydrogen-bonding network in complex borates: a neutron diffraction study of sulfoborite, Mg3[B(OH)4]2(SO4)(OH)F

Chemical composition (by EPMA-WDS) and crystal structure (by single-crystal neutron diffraction at 20 K) of sulfoborite from the type locality (Douglashall III potash mine, Westeregeln, Germany), were investigated. Chemical and structural data converge on the general mineral formula of this mineral: Mg3[B(OH)4]2(SO4)(OH)F. Potential substituents of Mg are Fe, Ca, K and Na, but their fraction is negligible (in total: 0.015 a.p.f.u). The measured concentration of S is consistent with that expected by stoichiometry (1 a.p.f.u.), and the neutron structure refinement confirms full site occupancy of the B site (namely, 2 a.p.f.u.). The fraction of F measured by EPMA-WDS approaches 1 a.pf.u. (i.e. , 0.953 a.p.f.u.), which mutually corroborates the finding based on the neutron diffraction experiment, ruling out a potential F vs. OH substitution. Chlorine, as a potential F substituent, is measured as below the EPMA-WDS detection limit, and therefore not significant. Excluding B, sulfoborite does not contain other industrially-relevant elements.

The neutron structure refinement confirms the general structural model of sulfoborite previously reported. The structure contains four basic building units: MgO2F(OH)3 and MgOF(OH)4 octahedra, and B(OH)4- and SO4-tetrahedra. Their combination generates a composite building unit, consisting of 3Mg-octahedra + 2B(OH)4-tetrahedra + 1SO4-tetrahedron: the three Mg-octahedra share an edge and/or corners, and are mutually connected by corner sharing to SO4- and B(OH)4-tetrahedra. The neutron refinement allowed a reliable location of five independent H sites, their libration regime (here described with an anisotropic model) and a clear picture of the H-bonding network, with details not previously reported in studies based on X-ray data. All the H sites are involved in hydroxyl groups, such that no H2O molecules occur in the crystal structure of sulfoborite. If we exclude one oxygen site (over nine unique O+F sites), all other ligands within the structure are potentially involved in H-bonding as donors or as acceptors, evidencing a pervasive network of interactions, mediated by the H sites, within the structure of sulfoborite.