DETERMINATION OF THE CONTENT AND DISTRIBUTION OF FIXED AMMONIUM IN ILLITE-SMECTITE BY X-RAY-DIFFRACTION - APPLICATION TO NORTH-SEA ILLITE-SMECTITE

A new X-ray diffraction method for the determination of the amount and distribution of fixed NH4+ in illite-smectite has been developed. Ill ite-smectite was saturated with K+ and heated at 150 degrees C. The 00 2 and 005 reflections were recorded with steps of 0.01 degrees 2 theta , and the experimental d values and the values for full-width at half- height (FWHH) were determined using a peak-profile-fitting procedure. Peak profiles were calculated with the NEWMOD program for illite struc tures having different amounts of NH4+ and different patterns for the distribution of NH4+ in interlayers. For Upper Jurassic illite-smectit e from North Sea oil source rocks, the amount and the distribution of NH4+ in illite interlayers were determined by comparing the experiment al values for d(005) and FWHH with the values calculated for the selec ted illite structures. The amounts of NH4+ determined in this manner c orrelate well with the amounts determined by an isotopic dilution meth od. The results demonstrate that these illite-smectite samples have K end-member illite and NH4 end-member illite (tobelite) layers and that the illite layers formed during diagenesis and oil generation actuall y are tobelite layers.