A single-crystal neutron diffraction study of berborite-2T, Be2(BO3)(OH)·H2O

The chemical composition (by EPMA-WDS and SIMS) and the crystal structure (by single-crystal X-ray diffraction at 293 K and neutron diffraction at 20 K) of berborite-2T from the Saga II quarry at Auenlandet, Larvik plutonic complex in southern Norway, were investigated. Chemical and crystallographic data confirm the general mineral formula of berborite: Be2(BO3)(OH)·H2O. F and Cl content (as potential substituents of the OH-group) was found to be below the detection limit of EPMA-WDS, and the average content of the other measured elements (i.e., Na, K, Ca, Mg, Fe, and S) is lower than 600 wt. ppm per equivalent oxide, and probably ascribable to micro-inclusions of other mineral species, rather than to elements replacing B or Be in berborite structure. Excluding B and Be, both considered as “critical elements” for the modern technology, berborite does not contain other industrially-relevant elements. The measured 11B/9Be ratio of berborite by SIMS was consistent with the expected value of the ideal formula (i.e., 1B : 2Be a.p.f.u). X-ray and neutron refinements confirm the previously reported general structural model of berborite-2T, built up by two layers made by regular (as governed by the 3-fold axes) corner-sharing triangular BO3-units and tetrahedral BeO3X (X = OH, H2O) units, mutually connected to give electron-neutral [(BO3)Be2X2]-layers. The X-groups (i.e., hydroxyls or H2O molecules) represent the out-of-plane apices of the tetrahedra, and the special position of the O sites on the triad implies a statistical occupancy of the X-groups (i.e., 1/3 OH and 2/3 H2O). Subsequent electron-neutral [(BO3)Be2X2]-layers are mutually connected only by H-bonds. The statistical occupancy of the H sites leads to a complex configuration of the H-bonding network, here described on the basis of the neutron diffraction data. The calculation of the difference-Fourier maps of the nuclear density function and the application of the Maximum Entropy Method to the experimental data consistently rule out dynamic disorder of the H sites, confirming the occurrence of a static disorder of the protons in the structure of berborite-2T, at least at 20 K.