Illite-smectites (I-S) in one Upper Jurassic mudstone core from East Greenland were investigated to determine their structural and crystal-chemical features and to find the relation between these features and source rocks. The phase composition and layer sequences were determined by X-ray diffraction (XRD), the distribution of octahedral cations over trans - and cis -octahedra by thermal analysis, the structural formulae by XRD, Mossbauer spectroscopy, and total chemical analysis, and the short-range order in isomorphous cation distribution by infrared (IR) and Mossbauer spectroscopies. For all samples (except one having maximum degree of ordering for R = 1), simulation of the experimental XRD patterns led to two different I-S models having indistinguishable diffraction patterns. For the first, single-phase model, expandability ( w S) is 0.45–0.60. For the second, two-phase model, two randomly interstratified I-S having w S equal to 0.40 and 0.85, respectively, are present in different proportions in different samples. The single-phase model was selected. A new approach for simulating the two-dimensional distributions of the isomorphous octahedral cations using IR and Mossbauer parameters revealed a tendency for Fe segregation into edge-shared octahedra that may form zigzag chains. Almost identical IR and Mossbauer parameters found for the I-S having different amounts of trans -vacant ( tv ) and cis -vacant ( cv ) layers (ranging from 0.08 to 0.80) demonstrate that these parameters are largely determined by local cation environments around Fe3+ and OH groups. Different levels in the Upper Jurassic Kimmeridgian core contain I-S having different structures. I-S of hemipelagic mudstones at the bottom (37 m depth) and in the middle (13 m depth) of the core, with a high proportion of cv layers and of smectite layers ( w S ~ 0.60), probably formed from volcanic material. The other four samples have a high proportion of tv layers and probably formed by weathering of micaceous material. One of these I-S, from a mudstone turbidite (at 27 m depth), having maximum degree of ordering for R = 1, probably originated from one type of parent rock, and three mudstones (at depths of 4, 12, and 13 m) with segregated I-S, probably originated from a second rock type.
Abstract Illite-smectite (I-S) mixed-layer minerals from North Sea oil fields and a Danish outcrop were investigated to determine the detailed structure and the diagenetic clay transformation. Clay layers in the chalk and residues obtained by dissolution of the chalk matrix at pH 5 were investigated. The phase compositions and layer sequences were determined by X-ray diffraction (XRD) including simulation with a multicomponent program. The structural formulae were determined from chemical analysis, infrared (IR) and 27 Al NMR spectroscopies and XRD, and the particle shape by atomic force microscopy (AFM). A high-smectitic (HS) I-S phase and a lowsmectitic (LS) illite-smectite-chlorite (I-S-Ch) phase, both dioctahedral, together constitute 80 – 90% of each sample. However, two samples contain significant amounts of tosudite and of Ch-Serpentine (Sr), respectively. Most of the clay layers have probably formed by dissolution of the chalk, but one Campanian and one Santonian clay layer in well Baron 2 may have a sedimentary origin. The HS and LS minerals are probably of detrital origin. Early diagenesis has taken place through a fixation of Mg in brucite interlayers in the LS phase, this solid-state process forming di-trioctahedral chlorite layers. During later diagenesis involving dissolution of the HS phase, neoformation of a tosudite or of a random mixed-layer trioctahedral chlorite-berthierine took place. In the tosudite, brucite-like sheets are regularly interstratified with smectite interlayers between dioctahedral 2:1 layers, resulting in ditrioctahedral chlorite layers.
Abstract The structure of 6-line and 2-line ferrihydrite (Fh) has been reconsidered. X-ray diffraction (XRD) curves were first simulated for the different structural models so far proposed, and it is shown that neither of these corresponds to the actual structure of ferrihydrite. On the basis of agreement between experimental and simulated XRD curves it is shown that Fh is a mixture of three components: (i) Defect-free Fh consisting of anionic ABACA . . . close packing in which Fe atoms occupy only octahedral sites with 50% probability; the hexagonal unit-cell parameters are a = 2-96 Å and c = 9-40 Å, and the space group is P 1c. (ii) Defective Fh in which Ac 1 Bc 2 A and Ab 1 Cb 2 A structural fragments occur with equal probability and alternate completely at random; Fe atoms within each of these fragments have identical ordered distribution with in the hexagonal super-cell with a = 5.26 Å. (iii) Ultradispersed hematite with mean dimension of coherent scattering domains (CSD) of 10-20 Å. The main structural difference between 6-line and 2-line Fh is the size of their CSD which is extremely small for the latter structure. Nearest Fe-Fe distances calculated for this new structural model are very close to those determined by EXAFS spectroscopy on the same samples.
Abstract For mixed-layer clay fractions from the North Sea and Denmark, X-ray diffractograms have been recorded for specimens saturated with Mg, Ca, Na and NH 4 , both airdry and intercalated with ethylene glycol, and the patterns have been computer-simulated with a multicomponent program. The mixed-layer fractions consist of an illite-smectite-vermiculite (I-S-V) phase constituting ~90% of the fraction and a kaolinite-illite-vermiculite (K-I-V) phase. For each I-S-V, the degree of swelling in swelling interlayers depends on both interlayer cation and glycolation, whereas the amount of non-swelling illite and swelling interlayers and the interstratification parameters are constant. Based on structural characteristics and the degree of diagenetic transformation, the samples investigated can be divided into three groups. The I-S-V of group one is predominantly detrital and has 0.69-0.73 illite, 0.26-0.20 smectite and 0.04-0.07 vermiculite interlayers, the illite, smectite and vermiculite interlayers being segregated. The I-S-V of group two has been diagenetically transformed and has 0.80 illite, 0.12 smectite and 0.08 vermiculite interlayers, the vermiculite interlayers being segregated whereas the illite and smectite have the maximum ordering possible for R = 1. The I-S-V of group three has been further transformed during diagenesis and has 0.84 illite, 0.08 smectite and 0.08 vermiculite interlayers. Statistical calculations demonstrate that the I-S-V transformation can be described as a single interlayer transformation (SIT) within the crystallites.
