X-Ray Powder Diffraction Identification of Illitic Materials

Abstract

The 10-Å clay components of sedimentary rocks (“illites”) are commonly mixtures of 100% nonexpandable illite and an ordered illite/smectite mixed-layer mineral. If the proportion of the illite/ smectite in a mixture is sufficient to produce a measurable reflection between 33–35°2θ (CuKα radiation) that is noncoincident with an illite reflection, the ratio of component layers and type of interstratification for the mixed-layer mineral can be determined. The identification technique developed in this study rests upon the following experimental findings for ordered illite/smectites of diagenetic origin: (1) the thickness of the illite layer in illite/smectites is 9.97 Å; (2) the thickness of smectite-ethylene glycol complex ranges from 16.7 to 16.9 Å; (3) illite/smectites form a continuous sequence of interstratification types—random, random/IS, IS, IS/ISII, ISII—and each type is related to a specific range of expandability.The new technique broadens the computer simulation method developed by R. C. Reynolds and J. Hower to include those sedimentary materials which are dominated by the presence of discrete illite, are low in illite/smectite, and, as such, have been described previously only by an “illite crystallinity index.”