Cation substitution in synthetic meridianiite (MgSO _4·11H _2O) II: Variation in unit-cell parameters determined from X-ray powder diffraction data

摘要

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 meridianiite-structured undecahydrates, melanterite- and epsomite-structured heptahydrates, novel enneahydrates and a new octahydrate. In this work we report the changes in unit-cell 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' unit-cell 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 _4·11H _2O to a very similar degree, yet the solubility limits inferred in our companion paper are >60 mol. % Co ~(2+) and <30 mol. % Zn ~(2+).