Original paper

Crystal chemistry of the kieserite–cobaltkieserite solid solution, Mg1-xCox(SO4)·H2O: well-behaved oddities

Bechtold, Andreas; Wildner, Manfred

Abstract

Earlier independent crystal-structure studies of kieserite, Mg(SO4)·H2O, and the isotypic transition-metal compounds Me2+ (SO4)·H2O (Me2+ = Mn, Fe, Co, Ni, Zn) indicated that the lattice parameters of kieserite are significantly larger than expected from both the respective ionic radii and the octahedral Mg/Me–O bond lengths. This volume mismatch was especially noticeable in comparison to the respective (high-spin) Co2+ compound with its only slightly larger ionic radius than Mg. In order to study this possible discrepancy, we synthesized several members of the kieserite–cobaltkieserite solid solution series, Mg1-x Co x (SO4)·H2O, and investigated the structures of ten representatives, including the end-members, using consistent single-crystal X-ray diffraction data. The present results confirm the supposed anomaly, showing a significant negative correlation of polyhedral vs. unit-cell volumes, which both change in total by 1.2 Å3over the series (V kies. = 356.0, V Co-kies. = 354.8 Å 3, = 2.078, = 2.097 Å). This effect is realized by folding and rotations within the polyhedral assemblage, apparently driven by angular O–Me–O and especially Me–O–S/Me changes. Apart from this oddity, the Mg1-x Co x (SO4) H2 O solid solution can be described as 'wellbehaved', i.e. no indications for miscibility gaps or cation ordering effects were found, and cell parameters and other structural data behave according to Vegard's law in linear relation with the refined Mg1-x Co x content. Explanations for the paradoxical volume behaviour of kieserite compared to cobaltkieserite (and other transition-metal kieserites) are probably related to the absence, respectively presence of partly filled 3 d orbitals and/or to well-established differences in electronegativity. Literature data on bond critical point properties of forsterite and transition-metal olivines indicate that the bond critical point r c lies significantly closer to Mg as compared to the transition cations, which might be ascribed in part to the absence of d orbitals at the Mg atom. In turn, the respective mutual influence on the electron density at the oxygen atoms in case of Mg–O bonds could represent a relevant modifier for the second coordination sphere and thus contribute to the paradoxical crystal chemical behaviour observed for kieserite as compared to isotypic transition-metal compounds in general and to cobaltkieserite in particular.

Keywords

magnesiumcobaltkieserite3d orbitalscrystal structurecobalttransition metalssulfatessolid solutionkieseritecrystal chemistry