Layered double hydroxides (LDHs) have been known for a considerable time and have been widely studied. The basic features of their structure, involving positively charged brucite-like layers together with charge-balancing anions and water in interlayer galleries are well understood, but some detailed aspects of their structure have been the subject of controversy in the literature. In this article we review the wide range of experimental and theoretical studies of the structure of LDHs, highlighting areas of consensus and currently unresolved issues. We focus on the range of composition for which LDHs may be formed, possible layer stacking polytypes, arrangement of guest species in the interlayer galleries and the extent of order-disorder phenomena, both long-range and short-range, in the layers and interlayer galleries.

Structural aspects of layered double hydroxides

Ab initio STO-3G molecular orbital theory has been used to calculate energy-optimized Si-O bond lengths and angles for molecular orthosilicic and pyrosilicic acids. The resulting bond length for orthosilicic acid and the nonbridging bonds for pyrosilicic acid compare well with Si-OH bonds observed for a number of hydrated silicate minerals. Minimum energy Si-O bond lengths to the bridging oxygen of the pyrosilicic molecule show a close correspondence with bridging bond length data observed for the silica polymorphs and for gas phase and molecular crystal siloxanes when plotted against the SiOSi angle. In addition, the calculations show that the mean Si-O bond length of a silicate tetrahedron increases slightly as the SiOSi angle narrows. The close correspondence between the Si-O bond length and angle variations calculated for pyrosilicic acid and those observed for the silica polymorphs and siloxanes substantiates the suggestion that local bonding forces in solids are not very different from those in molecules and clusters consisting of the same atoms with the same coordination numbers. An extended basis calculation for H4SiO4 implies that there are about 0.6 electrons in the 3d-orbitals on Si. An analysis of bond overlap populations obtained from STO-3G* calculations for H6Si2O7 indicates that Si-O bond length and SiOSi angle correlations may be ascribed to changes in the hybridization state of the bridging oxygen and (d – p) π-bonding involving all five of the 3d AO's of Si and the lone-pair AO's of the oxygen. Theoretical density difference maps calculated for H6Si2O7 show a build-up of charge density between Si and O, with the peak-height charge densities of the nonbridging bonds exceeding those of the bridging bonds by about 0.05 e A−3. In addition, atomic charges (+1.3 and −0.65) calculated for Si and O in a SiO2 moiety of the low quartz structure conform reasonably well with the electroneutrality postulate and with experimental charges obtained from monopole and radial refinements of diffraction data recorded for low quartz and coesite.

Ab initio calculated geometries and charge distributions for H 4 SiO 4 and H 6 Si 2 O 7 compared with experimental values for silicates and siloxanes

Structural Fe in clay minerals plays a large role in determining and influencing the chemical and physical properties and behaviour of this ubiquitous inorganic fraction of natural soils and sediments. Almost all clay minerals contain some Fe in their crystal structures, so the impact of Fe is great. Moreover, the effects of Fe are greatly amplified by its facile susceptibility to changes in oxidation state (redox modification), which are in fact larger than the effects of its mere presence in the mineral structure. Iron(III) is the more common oxidation state in the smectite structure under natural conditions, but bacterial activity in soils and sediments readily reduces it temporarily to Fe(II). In the laboratory, both sodium dithionite and bacteria were used extensively to effect reduction, followed by a plethora of subsequent characterizations. A variety of other reducing agents were also tried in the laboratory, with mixed results and with none of them being as effective as either dithionite or bacteria. Multiple redox cycles transform the smectite to a more illitic form, with both fixed K and residual Fe(II) increasing with each cycle. The surface reactivity of the smectites toward pesticides, chlorinated and various other organics, and redox-active metals is greatly increased once the structural Fe had been reduced. The cation exchange and fixation capacities increase, whereas specific surface area and swelling in water decrease. Recent studies also showed that reaction with redox-active anions (e.g. nitrate) is possible with polycations-exchanged, redox-modified smectites. Structural changes were characterized by a variety of chemical, instrumental, and spectroscopic methods, including the use of synchrotron X-ray sources, infrared, UV–visible, and scanning, environmental-sample, and high-resolution electron microscopy. In summary, Fe in smectites provides a robust means by which the properties and behaviour of soils and sediments change dynamically over space and time as environmental conditions fluctuate in response to global climate patterns and anthropogenic activity. Many opportunities await the curious and adventurous ones who decide to probe this phenomenon in order to advance understanding and uncover more avenues for meaningful exploitation.

Properties and Behaviour of Iron in Clay Minerals

This is a review of quantum chemical, statistical-mechanical, electrostatic, and crystal chemical studies of phyllosilicates. Atomic-level studies have examined the structure and bonding within phyllosilicate layers, the effects of cation substitution on bonding and charge distribution, the structure of water, and the distribution of cations in the interlayer, and the bonding between phyllosilicate layers. The major needs for future research are statistical-mechanical simulations of ion exchange reactions, electron transport in transition-metal-containing phyllosilicates, structure and bonding at phyllosilicate edges, and mechanisms of crystal growth and dissolution.

Atomic theories of phyllosilicates: Quantum chemistry, statistical mechanics, electrostatic theory, and crystal chemistry

9 – A Comparison of Experimental and Theoretical Bond Length and Angle Variations for Minerals, Inorganic Solids, and Molecules

A molecular-orbital study of distortions in the layer structures brucite, gibbsite and serpentine

An attempt has been made to characterize the stereochemical requirements of an aluminium atom coordinated by five hydroxyl groups (AlO5H52-) within the framework of CNDO/2 molecular orbital formalism. Total energy surfaces calculated for this system as the valence angles are varied within the constraints of C2v symmetry clearly show the 'reaction coordinate' for the so-called Berry pseudo- rotation mechanism. With all Al-O and O-H bond lengths fixed at 1.84 and 1.00 Ǻ respectively, the activation energy for trigonal-bipyramidal/square-pyramidal interconversion is 7.1 kJ mol-1. In addition, the minimum energy O(axial)-Al-O(basal) angle for the square-pyramidal group is estimated to be c. 103.5°. When one bond is lengthened relative to the other four the trigonal-bipyramidal configuration is further stabilized, but approaches tetrahedral geometry. When one bond is shortened square-pyramidal geometry is stabilized and the minimum energy O(ax)-Al-O(bs) angle increases. With two bonds lengthened (or shortened) the favoured trigonal-bipyramidal configuration is the one with long bonds directed towards the axial positions and at no stage is there a stabilization of square-pyramidal geometry. These results agree with trends in calculated Mulliken bond overlap populations and with observed ratios of axial-to-basal bond lengths in suggesting that for trigonal- bipyramidal stereochemistry the axial bonds are weaker, whereas for square-pyramidal geometry the axial bond is stronger. Moreover, consideration of the total energy for various isomers of Al(OH)4F2- indicates that more electronegative substituents prefer to enter the apical and basal positions in a trigonal bipyramid and a square pyramld respectively. For a cluster Al2(OH)82-, consisting of two aluminium trigonal-bipyramidal groups sharing a common edge of variable length, an energy minimum (62.2 kJ mol-1 lower than the undistorted case) is found at a shared edge length of 2.246 A, 13 % shorter than for the undistorted polyhedron. The corresponding observed distance for a similar dimer in the mineral andalusite, Al2SiO5, is 2.247(7) Ǻ. Plots of minimum energy shared edge length against mean bond length for various clusters M2(OH)8n-, where M = Al or Mg, clearly demonstrate an essentially linear relationship between the two parameters consistent wlth trends observed for natural systems in which M = Co, Cu, Mn and Zn.

R. C. Peterson

Papers

A molecular-orbital study of distortions in the layer structures brucite, gibbsite and serpentine

A Molecular Orbital Study of the Stereochemistry of Pentacoordinated Aluminium

A molecular orbital study of the stereochemistry of pentacoordinated aluminum