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THE ATOMIC STRUCTURE AND HYDROGEN BONDING OF DEUTERATED MELANTERITE, FeSO₄·7D₂O

Jennifer L. Anderson^1,, Ronald C. Peterson¹ and Ian P. Swainson²

¹ Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario K7L 3N6, Canada
² Neutron Program for Materials Research, National Research Laboratory, Chalk River, Ontario K0J 1J0, Canada

View larger version (58K): [in this window] [in a new window]   FIG. 1. Schematic diagram, with mineral names and formulae, showing the reversible hydration–dehydration phase changes that have been documented in M2+–SO4–H2O systems. Documented phase-changes are indicated by double headed arrows, and only the two minerals at arrow heads are involved in the hydration–dehydration reaction. Pathways may differ with metal substitution, changes in acidity, relative humidity, temperature or sample history.

View larger version (18K): [in this window] [in a new window]   FIG. 2. Neutron-diffraction patterns of deuterated melanterite, = 1.3308 and 2.3731 Å. Observed data are shown as crosses, the fitted model as a straight line, and the difference (obs.–calc.) is displayed below.

View larger version (74K): [in this window] [in a new window]   FIG. 3. Schematic representation of the atomic structure of melanterite (ATOMS 6.0, Dowty 1995). The undulating layer-structure parallel to the (001) perfect cleavage is illustrated here and is indicated by the dashed black line. Chains of M1 octahedra (green) are linked to chains of M2 octahedra (blue) via H-bonds to chains of sulfate tetrahedra (yellow), creating a layer of repeating chains of SO4–M1–SO4–M2 polyhedra. The interstitial H2O molecule is H-bonded between the M2 octahedra and the SO4 tetrahedra in the melanterite structure. The layer structure is bridged by four unique H-bonds per formula unit.

View larger version (56K): [in this window] [in a new window]   FIG. 4. Details of the M–SO4–M polyhedron linkages within the layer structure of minerals in the melanterite and epsomite groups (ATOMS 6.0). (a) The M1 octahedra in melanterite-group minerals are linked to the SO4 tetrahedra in a pseudo-edge- and face-sharing arrangement of H-bonds. (b) The M2 and SO4 polyhedra in melanterite-group minerals are linked via the interstitial H2O molecule and a pseudo-edge-sharing arrangement of H-bonds. (c) In epsomite-group minerals, six of the seven H-bonds within the layer structure are linked in a pseudo-edge-sharing arrangement between the M2+ octahedra and SO4 tetrahedra.

View larger version (19K): [in this window] [in a new window]   FIG. 5. Plot of H–O and H-bond distances in the H2O molecule. Small closed circles and line of least-squares fit represent data from Ferraris & Franchini-Angela (1972). Small open circles represent data from the following papers: Angel & Finger (1988), Bacon & Titterton (1975), Bargouth & Will (1981), Baur (1962, 1964a, b, 1967), Baur & Rolin (1972), Blake et al.(2001), Calleri et al.(1984), Elerman (1988), Gerkin & Reppart (1988), Hawthorne et al.(1987), Held & Bohaty (2002), Iskhakova et al.(1991), Kellersohn (1992), Kellersohn et al.(1991), Ptasiewicz-Bak et al.(1993), Wildner & Giester (1991), Zahrobsky & Baur (1968), Zalkin et al. (1967). The solid circles and linear-regression line represent the birfurcated and non-bifurcated H-bonds from the survey of neutron-diffraction studies (Ferraris & Franchini-Angela 1972). Hydrogen bonds determined by X-ray diffraction are significantly shorter than those determined by neutron diffraction.

View larger version (28K): [in this window] [in a new window]   FIG. 6. Schematic diagram of the network of H-bonds in melanterite, as determined by the combined-histogram powder neutron-diffraction refinement of deuterated melanterite, FeSO4·7D2O. The O–H distances (Å) are indicated by the dashed lines. Dotted lines represent H-acceptor bonds. This diagram is not to scale, and only represents the topology of the atomic linkages present in the structure. In melanterite, the apical H2O molecules (Ow6) of the M2 octahedra are the only two H2O molecules, within a M2+ coordination sphere, to receive additional H-bonds from neighboring H atoms. The Fe2–Ow6 bond lengthens to 2.169(10) Å to compensate for the reduced bond-valence due to this additional H-bond interaction.

View larger version (39K): [in this window] [in a new window]   FIG. 7. Hydrogen bonding schemes for (a) orthorhombic heptahydrate structures of the epsomite group, (b) retgersite, a tetragonal hexahydrate, (c) monoclinic heptahydrate structures of the melanterite group, and (d) monoclinic hexahydrate minerals of the hexahydrite group. These diagrams show the distinction between the M2+ ions in octahedral coordination with six H2O molecules that receive two H bonds from external H2O molecules and those that receive three. Minerals of the general formula M2+SO4·nH2O, where n = 5, 4, 3, 1, contain octahedra formed by less than six H2O molecules and were not considered in this discussion.

View larger version (82K): [in this window] [in a new window]   FIG. 8. Schematic representation of the (010) perfect cleavage in epsomite (ATOMS 6.0, Dowty 1995). The cleavage plane is indicated by a solid black undulating line. The H-bonds associated with the interstitial H2O molecule bridge the layers of octahedra linked to sulfate tetrahedra. This layer in epsomite has a tighter, more corrugated topology than the similar layer of the melanterite structure.