Structural investigations of mixed-layer smectite-illite clay minerals from North Sea oil source rocks

The purpose of the present investigation is to identify mixed-layering between illite and smectite in oil source rocks with HREM and electron diffraction. Smectite and mica are both groups within the 2:1 type of layer silicates. Illite is a commonly used term for micas in the fraction less than 2 microns. Illite has a fixed d(001) spacing of 10A. Smectite is able to swell from 10A to more than 18A under uptake of water or organic polar molecules. Mixed-layer illite-smectite is built of alternating layers of illite and smectite. Smectite and illite layers are recognized by x-ray diffraction (XRD) from their swelling characteristics, but cannot be distinguished in this way in HREM, because smectite layers tend to contract to 10A by evaporation of water in the vacuum of the microscope. Illites are identified by HREM as small packets showing very regular lattice fringes and smectites as highly imperfect, anastomosing layers showing very irregular lattice fringes (1).