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Research Paper |
1 Clay Mineralogical Laboratory, Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark, 2 Institute of Geology, Russian Academy of Science, Pyzhevsky per. D7, 109017 Moscow, Russia, and 3 Instrument Centre for Solid-State NMR Spectroscopy, Department of Chemistry, University of Aarhus, DK-8000 Aarhus C, Denmark
* E-mail: hl{at}geus.dk
(Received 21 January 2002; revised 27 May 2002)
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-smectitic (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 di-trioctahedral chlorite layers.
KEYWORDS: illite-smectite, chlorite-smectite, chalk, aeolian dust, North Sea, Cretaceous, Tertiary, X-ray diffraction, Nuclear Magnetic Resonance Spectroscopy
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