Mixed-layer kaolinite-illite-vermiculite in North Sea shales

The finest fractions of Upper Jurassic shales from the North Sea and onshor e Denmark contain 80-90% of an illite-smectite-vermiculite (I-S-V) mixed-la yer mineral and, in addition a phase which has X-ray diffraction (XRD) peak s at 7.20-7.26 Angstrom and 3.56-3.58 Angstrom in air-dried and glycolated specimens. This phase may be a fine kaolinite with a small thickness of coh erent scattering domains (CSDs) or alternatively a mixed-layer mineral whic h has kaolinite as the dominant component. For one sample from the Norwegia n well 9/4-3, these alternatives are investigated using the multi-specimen method by which agreement between the experimental pattern and the pattern calculated for one and the same structure is obtained for each of several s pecimens saturated with different cations and with/without glycolation. It is demonstrated that the modelled XRD patterns for a kaolinite-illite-vermi culite (K-I-V) structure having 0.94 kaolinite, 0.03 illite and 0.03 vermic ulite layers and random alternation fit the experimental patterns.