The structure and diagenetic transformation of illite-smectite and chlorite-smectite from North Sea Cretaceous-Tertiary chalk

H. Lindgreen, V.A. Drits, B.A. Sakharov, H.J. Jakobsen, A.L. Salyn, L.G. Dainyak, H. Krøyer

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51 Citations (Scopus)


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 27Al NMR spectroscopies and XRD, and the particle shape by atomic force microscopy (AFM). A high-smectite (HS) I-S phase and a low-smectitic (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 ditrioctrahedral chlorite layers.

Original languageEnglish
Pages (from-to)429-450
Number of pages22
JournalClay Minerals
Issue number3
Publication statusPublished - Sept 2002


  • Aeolian dust
  • Chalk
  • Chlorite-smectite
  • Cretaceous
  • Illite-smectite
  • North Sea
  • Nuclear Magnetic Resonance Spectroscopy
  • Tertiary
  • X-ray diffraction

Programme Area

  • Programme Area 4: Mineral Resources


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