Diagenetic evolution of crystallite thickness distribution of illitic material in Carpathian shales, studied by the Bertaut-Warren-Averbach XRD method (MudMaster computer program)

The MudMaster computer program, based on the modified Bertaut-Warren-Averba ch (BWA) XRD method, was used in the study of diagenetic evolution of illit ic material in shales. The illitic material was characterized by XRD as a m ixture of illite-smectite (T-S) and discrete illite. The experimental condi tions for complete dehydration of swelling clays, necessary in this method, were established for a climate of high relative humidity (RH >40%). It was found that the distribution of crystallite thickness of dehydrated illitic material of shales is described by the lognormal law, as was established e arlier by the BWA method for pure I-S from pyroclastic rocks. The parameter s characterizing this distribution evolve with depth: the mean crystallite thickness (Te), the distribution broadening (DW) and the parameter of a log normal distribution (beta(2)) increase, whereas the percentage of the most frequent crystallite thickness decreases. The observed scatter of values is not random but indicates fluctuations in the characteristics of the illiti c material of shales. The results imply that the modified Scherrer techniqu e of measuring mean crystal thickness from the broadening of XRD reflection s can be extended at least to some shales.