A multi-methodological study of kurnakovite : a potential B-rich aggregate

The crystal structure and crystal chemistry of kurnakovite from Kramer Deposit (Kern County, California), ideally MgB3O3(OH)5·5H2O, were investigated by single-crystal neutron diffraction (data collected at 293 and 20 K) and by a series of analytical techniques aimed to determine its chemical composition. The concentration of more than 50 elements was measured. The empirical formula of the sample used in this study is Mg0.99(Si0.01B3.00)Σ3.01O3.00(OH)5·4.98H2O. The fraction of rare earth elements (REE) and other minor elements are, overall, insignificant. Even the content of fluorine, as a potential OH-group substituent, is insignificant (i.e., ~0.008 wt%). The neutron structure model obtained in this study, based on intensity data collected at 293 and 20 K, shows that the structure of kurnakovite contains: [BO2(OH)]-groups in planar-triangular coordination (with the B-ions in sp2 electronic configuration), [BO2(OH)2]-groups in tetrahedral coordination (with the B-ions in sp3 electronic configuration), and Mg(OH)2(H2O)4-octahedra, connected into (neutral) Mg(H2O)4B3O3(OH)5 units forming infinite chains running along [001]. Chains are mutually connected to give the tri-dimensional structure only via hydrogen bonding, and extra-chains “zeolitic” H2O molecules are also involved as “bridging molecules.” All the oxygen sites in the structure of kurnakovite are involved in hydrogen bonding, as donors or as acceptors. The principal implications of these results are: (1) kurnakovite does not act as a geochemical trap of industrially relevant elements (e.g., Li, Be, or REE), (2) the almost ideal composition makes kurnakovite a potentially good B-rich aggregate in concretes (for example, used for the production of radiation-shielding materials for the elevated ability of 10B to absorb thermal neutrons), which avoids the risk to release undesirable elements, for example sodium, that could promote deleterious reactions for the durability of cements.