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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 (NH 4 ,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 H 2 O content]. The concentrations of more than 50 elements were measured. The experimental formula of the boussingaultite is: [(NH 4 ) 1.77 K 0.22 ) Σ1.99 [(Mg 0.95 Mn 0.06 ) Σ1.01 (H 2 O) 5.7 ](SO 4 ) 1.99 . Neutron data analysis confirms that the structure of boussingaultite is built up by isolated Mg(H 2 O) 6 - octahedra, along with isolated NH 4 - and SO 4 -tetrahedra connected by a complex H-bonds network. Mg 2+  is completely solvated by H 2 O 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 H 2 O molecules, bonded to Mg, is in line with that usually observed in crystalline compounds. The H 2 O molecules show moderate-strong H-bonds, with H···O acceptor  and O donor ···O acceptor  ranging between 1.72–1.87 and 2.70–2.84 Å, respectively, along with O donor -H···O acceptor  angles between 168–178°. The four independent N-H···O bonds show H···O acceptor  and N donor ···O acceptor  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 H 2 O molecules and of the NH 4 -group involve the oxygen sites of the SO 4 -group as acceptors: the SO 4 -group is, therefore, the “bridging unit” between the NH 4  and the Mg(H 2 O) 6  units, via H-bonds. Our structure refinement proved, unambiguously, that the partial K4 +  vs. NH +  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. NH +  replacement implies 4 a change in the configuration of the SO 4 - 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 Mn 2+  replaces Mg 2+  at the Mg site. No evidence of distortion of the octahedron is observed in response to such a replacement, but the fraction of Mn 2+  is modest. An analysis of previous Raman and IR results is provided, and is compared with the experimental results of this study.

A neutron diffraction study of boussingaultite, (NH 4 ) 2 [Mg(H 2 O) 6 ](SO 4 ) 2