A neutron diffraction study of the hydrous borate inderborite, CaMg[B3O3(OH)5]2(H2O)4•2H2O

The crystal chemistry of inderborite, a B-rich mineral (B2O3 similar to 41 wt%) with ideal formula CaMg[B3O3(OH)(5)](2).6H(2)O or CaMg[B3O3(OH)(5)](2)(H2O)(4)(H2O)(4).2H(2)O from the Inder Deposit, Kazakhstan, was re-investigated by a multi-methodological approach (single-crystal X-ray and neutron diffraction, electron probe micro-analysis in wavelength-dispersive mode, laser ablation multi-collector inductively mass spectrometry). The experimental findings show that the real chemical formula of inderborite from the Inder Deposit is virtually identical to the ideal one: the fraction of potential isomorphic substituents is insignificant. Boron is, therefore, the only industrially relevant element occurring in this mineral. The in situ B isotope composition of the Inder inderborite shows enrichment in the heavy B-11 isotope, giving a weighted mean delta B-11(NIST951) of +35.15 +/- 0.49 parts per thousand (2 sigma, N = 6). Such a positive delta B-11 value falls within the range of values in which the source of boron is ascribable to marine reservoirs rather than to terrestrial ones. X-ray (at 293 K) and neutron (at 20 K) structure refinements confirm that the principal building block unit of the structure is a [B3O3(OH)(5)](2-) ring, consisting of two BO2(OH)(2) tetrahedra (B-ion in sp(3) electronic configuration) and one planar-triangular BO2OH group (B-ion in sp(2) electronic configuration). In the [B3O3(OH)(5)](2-) ring, all the oxygen atoms that are not shared between two boron atoms are protonated. The building units share corners with the CaO2(OH)(4)(OH2)(2) polyhedra and Mg(OH)(4)(OH2)(2) octahedra, forming hetero-polyhedral sheets parallel to (100). Subsequent hetero-polyhedral sheets are mutually connected only by H-bonding interactions, even mediated by the zeolitic ("interstitial") H2O molecules. Ten out of 11 independent oxygen sites in the structure of inderborite are involved in H-bonds as donors or acceptors, and this reflects the pervasive effect of the H-bonding network. The role played by the complex H-bond network is expected to be substantial on the stability of the crystalline edifice, having effects within the single hetero-polyhedral sheet, between subsequent sheets, and in the bonding with the interstitial zeolitic H2O molecules. Finally, the potential utilizations of inderborite, as a B-bearing mineral, are discussed.