A new X-ray diffraction method for the determination of the amount and distribution of fixed NH4+ in illite-smectite has been developed. Illite-smectite was saturated with K+ and heated at 150 °C. The 002 and 005 reflections were recorded with steps of 0.01° 2θ, 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 structures having different amounts of NH4+ and different patterns for the distribution of NH4+ in interlayers. For Upper Jurassic illite-smectite from North Sea oil source rocks, the amount and the distribution of NH4+ in illite interlayers were determined by comparing the experimental values for d005 and FWHH with the values calculated for the selected illite structures. The amounts of NH4+ determined in this manner correlate well with the amounts determined by an isotopic dilution method. 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 actually are tobelite layers.

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