A neutron diffraction study of boussingaultite, (NH4)2[Mg(H2O)6](SO4)2

The crystal structure and chemical composition of boussingaultite from Pécs-Vasas, Mecsek Mountains, South Hungary, were investigated by single-crystal neutron diffraction (at 20 K) along with a series of chemical analytical techniques [i.e., gravimetric determination of sulfates, EDTA titrimetric determination of magnesium, ion selective electrode for F and Cl, indirect gravimetric determination of ammonium as (NH4,Rb,Cs,K) tetraphenylborate, inductively coupled plasma atomic emission spectroscopy for REE and other minor elements, elemental analysis for C, N, and H content, high-T mass loss for H2O content]. The concentrations of more than 50 elements were measured. The experimental formula of the boussingaultite is: [(NH4)1.77K0.22)Σ1.99[(Mg0.95Mn0.06)Σ1.01(H2O)5.7](SO4)1.99. Neutron data analysis confirms that the structure of boussingaultite is built up by isolated Mg(H2O)6- octahedra, along with isolated NH4- and SO4-tetrahedra connected by a complex H-bonds network. Mg2+ is completely solvated by H2O molecules in a typical octahedral bonding configuration. All the seven independent oxygen sites in the structure are involved in H-bonds, as donors or as acceptors. The geometry of all the H2O molecules, bonded to Mg, is in line with that usually observed in crystalline compounds. The H2O molecules show moderate-strong H-bonds, with H···Oacceptor and Odonor···Oacceptor ranging between 1.72–1.87 and 2.70–2.84 Å, respectively, along with Odonor-H···Oacceptor angles between 168–178°. The four independent N-H···O bonds show H···Oacceptor and Ndonor···Oacceptor distances ranging between 1.81–2.00 and 2.84–2.98 Å, respectively, with N-H···O angles between 158–176°. All the H-bonds of the H2O molecules and of the NH4-group involve the oxygen sites of the SO4-group as acceptors: the SO4-group is, therefore, the “bridging unit” between the NH4 and the Mg(H2O)6 units, via H-bonds. Our structure refinement proved, unambiguously, that the partial K+ vs. NH4 + replacement generates a local disorder. K lies at the N site, and its bonding configuration can be described by a distorted polyhedron with CN = 8. However, the K+ vs. NH4 + replacement implies a change in the configuration of the SO4- tetrahedron, through a sort of rotation of the polyhedron. This is the first evidence of the presence of a partial picromerite component in the boussingaultite structure, which gives rise to a local disorder likely due to the significantly different bonding configurations of the two cations. Our refinement also revealed that Mn2+ replaces Mg2+ at the Mg site. No evidence of distortion of the octahedron is observed in response to such a replacement, but the fraction of Mn2+ is modest. An analysis of previous Raman and IR results is provided, and is compared with the experimental results of this study.