The distributions of illite crystal (‘fundamental particle’) thickness in <0.2 μm fractions of 13 shale samples (from the Carpathian Foredeep, Poland), obtained using the Bertaut-Warren-Averbach X-ray diffraction method (MudMaster computer program), were analyzed and interpreted in terms of the mechanism of smectite illitization. All illite crystal thickness distributions in the analyzed shales are characterized by an ‘asymptotic’ shape. The origin of the asymptotic-type distributions is explained by the heterogeneity of illite crystals in shales, i.e. superposition of different populations of crystals, those included in illite-smectite (I-S) interstratifications, and those which occur as discrete illite. The analysis of the distributions in shales shows that the most frequent thickness class of illite crystals forming I-S is 2 nm. Discrete illite is composed of thicker crystals; though crystals as thin as 2 nm can also contribute to this population. The modeling of asymptotic distributions in shales by using a number of theoretical lognormal distributions shows that with progressive diagenesis, the mean thickness of illite crystals forming the I-S component increases gradually, whereas the discrete illite does not show a clear trend.
The diverse origins of illite crystals in shales do not permit determination of the mechanism of smectite illitization from the evolution of the shape of the overall crystal-thickness distribution during diagenesis. Therefore, in order to understand the mechanism of smectite illitization in shales, an attempt was made to trace the relative gains and losses of crystals of different thicknesses during illitization. This approach indicates the following mechanism of smectite illitization in the investigated shales: dissolution of smectite monolayers accompanied by growth of 2 nm crystals which are largely of detrital origin. Nucleation of 2 nm illites seems to be very limited.