Cation substitution in synthetic meridianiite (MgSO4·11H2O) II: variation in unit-cell parameters determined from X-ray powder diffraction data

Abstract

We have prepared aqueous MgSO 4 solutions doped with various divalent metal cations (Ni 2? , Zn 2? , Mn 2? , Cu 2? , Fe 2? , and Co 2? ) in proportions up to and including the pure end-members. These liquids have been solidified into fine-grained polycrystalline blocks of metal sulfate hydrate ? ice by rapid quenching in liquid nitrogen. In a companion paper (Fortes et al., in Phys Chem Min 39) we reported the identification of various phases using X-ray powder diffraction, including meridianiitestructured undecahydrates, melanterite-and epsomitestructured heptahydrates, novel enneahydrates and a new octahydrate. In this work we report the changes in unitcell parameters of these crystalline products where they exist over sufficient dopant concentrations. We find that there is a linear relationship between the rate of change in unit-cell volume as a function of dopant concentration and the ionic radius of the dopant cation; large ions such as Mn 2? produce a substantial inflation of the hydrates' unitcell volume, whereas smaller ions such as Ni 2? produce a modest reduction in unit-cell volume. Indeed, when the data for all hydrates are normalised (i.e., divided by the number of formula units per unit-cell, Z, and the hydration number, n), we find a quantitatively similar relationship for different values of n. Conversely, there is no relationship between the degree of unit-cell inflation or deflation and the limit to which a given cation will substitute into a certain hydrate structure; for example, Co 2? and Zn 2? affect the unit-cell volume of MgSO 4 Á11H 2 O to a very similar degree, yet the solubility limits inferred in our companion paper are [60 mol. % Co 2? and \30 mol. % Zn 2? .

Cation substitution in synthetic meridianiite (MgSO4·11H2O) I: X-ray powder diffraction analysis of quenched polycrystalline aggregates

“…Further studies of doped single-crystal and powder specimens are underway using neutron diffraction and optical emission spectroscopy in an effort both to understand finer details of the phenomena reported in this work and to determine the partitioning of foreign cations into the Mg11 crystal under conditions of equilibrium crystallisation. Studies of oxyanion substitution have already revealed solid solution between MgSO 4 Á11H 2 O, MgCrO 4 Á11H 2 O and MgSeO 4 Á11H 2 O end members in quenched aqueous solutions (Fortes and Wood 2012), and we have been able to prepare single crystals of the chromate analogue from aqueous solution at -11°C. In the following paper in this volume, the data reported here are analysed further to elucidate the anisotropic elastic strains induced by cation doping in rapidly quenched aqueous solutions.…”

“…Given the isolated nature of the M 2? (H 2 O) 6 octahedra in the Mg7 and Mg11 structures (Fortes et al 2006(Fortes et al , 2008Peterson and Wang 2006), it seemed likely that any distortion of these octahedra brought about by the d-orbital structure of the transition metal cations could be 'absorbed' by the extensive hydrogen bond network. In fact, however, we observed that only MgSO 4 forms an undecahydrate in quenched aqueous solutions, which leads us to conclude that stable endmember undecahydrates of ZnSO 4 , NiSO 4 , CuSO 4 , CoSO 4 , MnSO 4 , and FeSO 4 cannot form.…”

“…X-ray powder diffraction X-ray powder diffraction data were collected on a PANalytical X'Pert Pro multipurpose powder diffractometer (using germanium monochromated Co Ka 1 radiation and X'Celerator multi-strip detector) equipped with a custom-made thermoelectrically cooled cold stage that is described in detail by Wood et al (2012). Diffraction patterns were integrated in steps of 0.0167°2h; the active length of the X'Celerator detector (2.122°) means that dwell times of between 25 and 450 s per step translate into overall measurement times between 16.7 min and 5 h; shorter scans were typically used for rapid phase identification whilst the cold stage thermally equilibrated and longer scans were made once the temperature had stabilized.…”

“…liquid. As a result, although discovered over 175 years ago (Fritzsche 1837), this substance has received scant attention until relatively recently, when its structure and properties were determined (Peterson and Wang 2006;Fortes et al 2008). Interestingly, species which may be metastable in aqueous solution in one system (e.g.…”

MgSO4·11H2O and MgCrO4·11H2O based on time-of-flight neutron single-crystal Laue data

“…The eigenvalues (principal axis magnitudes) and eigenvectors (direction cosines) of the elastic strain tensor derived from the observed changes in unit-cell parameters, obtained using the method described by Fortes et al (2012b), are listed in Table 3. A Reynolds glyph is used to depict the representation surface of this tensor in Fig.…”