A multi-methodological study of kernite, a mineral commodity of boron

Kernite, ideally Na2B4O6(OH)2·3H2O, is one of the major constituent of the borate deposits and one of the most important mineral commodity of B. The chemical composition and the crystal structure of kernite from the Kramer Deposit (Kern County, California) were investigated by a series of analytical techniques (i.e., titrimetric determination of B content, gravimetric method for Na, ion selective electrode for F, high-T mass loss for H2O content, inductively coupled plasma atomic emission spectroscopy for REE and other minor elements, elemental analysis for C, N and H content) and by single-crystal X-ray (at 293 K) and neutron (at 20 K) diffraction. The concentration of more than 50 elements was measured. The general experimental formula of the kernite sample used in this study is: Na1.99B3.99O6(OH)2·3.01H2O. The fraction of other elements is, overall, insignificant: excluding B, kernite from the Kramer Deposit does not act as geochemical trap of other technologically-relevant elements (e.g., Li, Be or REE). The X-ray and neutron structure model obtained in this study confirm that the structure of kernite is built up by: two (crystallographically independent) triangular BO2OH groups and two tetrahedral BO4 groups, which share corner-bridging O atoms to form 3-fold rings, giving chains running along [010], and NaO4(OH)(OH2) and NaO2(OH)(OH2)3 polyhedra. Positional disorder of two H sites of H2O molecules was observed by the neutron structure refinement and corroborated by the maximum-entropy method calculation, which consistently provided a model based on a static disorder, rather than a dynamic one. The H-bonding network in the structure of kernite is complex, pervasive and play a primary role on its structural stability: the majority of the oxygen sites are involved in H-bonding, as donors or as acceptors. The potential utilizations of kernite, as source of B (B2O3 ~ 50wt%), are discussed, on the basis of the experimental findings of this study.