Synthetic Fe 2+ monosulfide, FeS am , displays a disordered tetragonal mackinawite structure. It is nanocrystalline, with an average primary particle size equivalent to a crystallite size of 4 nm and a corresponding specific surface area of 350 m 2 /g. It can be described in terms of a mixture of two end-member phases with different long-range ordering, which we refer to as MkA and MkB. MkA has an average primary particle size of 2.2 ¥ 1.7 nm and lattice parameters a = b = 4.0 Å, c = 6.6 ± 0.1 Å. MkB has an average primary particle size of 7.4 ¥ 2.9 nm and lattice parameters a = b = 3.7 Å, c = 5.5 ± 0.2 Å. A typical disordered mackinawite precipitate consist of 30% MkA and 70% MkB and the proportion of MkA decreases with age. Lattice expansions relative to crystalline mackinawite (a = b = 3.7 Å, c = 5.0 Å) may be explained by intercalation of water molecules between the tetrahedral sheets and by lattice relaxation due to small crystallite size.The formation of two phases of FeS am is consistent with competing pathways involved in its formation from aqueous solution. MkA may be equivalent to sheet-like precipitated aqueous FeS clusters. The reactivity of FeS am is dependent on the proportion of the two end-member phases. These in turn are dependent on the conditions of formation, especially pH, and the age of the precipitate. These observations partly explain the reported differences in FeS am reactivity in experimentation and in the environment. The structural model has implications for the behavior of natural acid volatile sulfides in scavenging elements from solution in natural environments.

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Differential X-ray powder diffraction (DXRD) and extraction procedures were used to characterize the iron oxides present in four sediments from contrasting environments in the North Sea. Stations were located in depositional areas on the southern shelf (German Bight) and on the north-eastern shelf-slope transition (Skagerrak) and in areas with no net deposition in the southern North Sea. Poorly crystalline ferrihydrite and akageneite (extractable with 0.1 M HCI and 0.2 M NH,-oxalate) were identified in the fine sediment fraction ( < 10 pm) of surface samples at all locations. Evidence for the dominant role of these Fe oxides in the binding of phosphorus in North Sea sediments was obtained from the good relationship of both the content of Fe-bound P and the linear adsorption coefficient for phosphate with NH,-oxalate extractable Fe. A tight coupling of pore water Fe*' and HPO$-was observed at 3 stations. Pore water Fe'+/HPOjratios at maximum pore water concentrations of Fe*+ were similar to NH,oxalate Fe/Fe-bound P ratios for surface sediment at these locations, and were in the range known for synthetic poorly crystalline Fe oxides. This suggests that pore water HPOi-production at the time of core collection was dominated by release from poorly crystalline Fe oxides. In contrast, at the German Bight station, much higher HPOi-levels and a decoupling of pore water Fe*+ and HPOi-was observed, suggesting a larger contribution of mineralization of organic matter to pore water HF'Oa-than at the other sites. Solid phase P analyses indicate possible redistribution of Fe-bound P to another inorganic phase at depth at the Skagerrak station, but not at the other stations. The persistence with depth of poorly crystalline Fe oxides and Fe-bound P suggests that these Fe phases can act as both a temporary and permanent sink for P in continental margin sediments.

Raman microscopy of the kaolinite polymorphs was used to study single crystals and bundles of aligned crystals of kaolinite. The spectra of the hydroxyl stretching region were both sample and orientation dependent. Kaolinites can be classified into two groups according to the ratio of the intensities of the 3685 and 3695 cm -I bands. No relationship was found between the d-spacing and the crystal domain size measurement from the 001 reflection and the Raman spectral intensities indicating the Raman spectra are independent of d-spacing and crystallinity. However, a relationship of the crystallinity in the a-b direction and intensities of the 3685 and 3695 cm -1 bands indicate that the relative position of one layer to the other determines the position of the inner surface hydroxyl groups and the hydrogen bonding with the oxygen of the opposite layer. A new hypothesis based on symmetric and non-symmetric hydrogen bonding of the inner surface hydroxyl groups is proposed to explain the two inner surface hydroxyl bands centred at 3685 and 3695 cm -1, The bands at 3670 and 3650 cm -1 are described in terms of the out-of-phase vibrations of the in-phase vibrations at 3695 and 3685 cm -1.Dispersive Raman spectroscopy has been used to a limited extent in the study of the kaolinite clay minerals (Wiewiora, 1979;Johnston et al., 1985;Michaelian, 1986). Wiewiora first identified Raman bands for kaolinite at 3620, 3650, 3667 and 3682 cm -1 with a prominent shoulder at 3692 cm -~. Tentative assignments of these bands were made in terms of the inner and inner surface hydroxyl groups. The conclusion was reached that the band assignments were attributable to individual OH oscillators rather than coupled OH units. Johnston et al. (1985) showed that the dependence of the hydroxyl .stretching frequencies of the aqueous suspension were pH dependent. Kaolinites of different origin were shown to have different spectra. Hydroxyl stretching frequencies were found at 3621,3652, 3668, 3688 and 3696 cm -j. Johnston et al. (1985) suggested that the Raman technique offered an excellent tool for the characterization of the hydroxyl groups in natural kaolinites. Michaelian (1986) proposed likely origins of the 3684 cm -~ band in terms of uncoupled inner surface hydroxyl stretching and transverse longitudinal splitting.Fourier transform (FT) Raman spectroscopy has been shown to be very useful for the study of the kaolinite clay mineral hydroxyl groups (Frost, 1995(Frost, , 1997 Frost et aI. 1993). In this work, detailed studies of the hydroxyl stretching frequencies Of the kaolinite polymorphs were reported. In particular, the differences in the Raman spectra of ordered kaolinite (KGa-1) and a disordered kaolinite (KGa-2) were elucidated. Although the technique of Raman microscopy has been in existence for a considerable length of time (Dhamelincourt et al., 1979;Dubessy et al., 1982), the application of the technique to the study of clay mineral structure has been limited. The advent of low-energy near-IR lasers and the use of cha...

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