Calculated X-ray diffraction profiles of 10 A-17 A interstratified clay minerals, which make allowance for fundamental illite particles, confirm many of the inferences made in a previous analysis by Corbat6 & Tettenhorst (1987). However, some of these profiles do not appear to match well with patterns obtained from real interstratified clay minerals. The lack of agreement is apparently a consequence of the assumption that the interlayer spacing formed at the contact of I and S (illite and smectite) silicate layers is exclusively 17 A.. The results suggest that this interlayer spacing may be either 10 A or 17 A.Comparison of calculated X-ray diffraction (XRD) profiles of interstratified 10 A-17 /k (hereafter termed I-S) clay minerals with patterns obtained from real clays has improved the understanding of these clays and has provided insight into their origin and behaviour. The Reynolds/MacEwan methods of calculating XRD profiles (Reynolds, 1980) have played a significant role in the interpretation of diffraction patterns of I-S clay minerals. The proportions of illite and smectite, presence and kind of order, and the range of particle thicknesses can be inferred when a good match is obtained between a calculated XRD profile and an XRD pattern from a real clay. Variations in the XRD patterns in sequences of samples have allowed investigators to assess the effect of temperature, time, burial depth, proximity to igneous bodies, ion mobility, and ion competition in the transformation of smectite to illite. Corbat6 & Tettenhorst (1987) formulated a different approach to the analysis of I-S clay minerals. The objective of that study was to try to provide additional insight by incorporating the concepts of fundamental illite particles and expandable interfaces (interlayers) between illite particles (McHardy et al., 1982; Nadeau et al., 1984a,b). This approach has the potential to allocate the expandable portion of these interstratified clays between smectite and illite. The purpose of the present study is to develop a procedure to calculate XRD profiles of I-S clays based on the parameters set forth in the previous analysis. FUNDAMENTAL PARTICLESFundamental illite particles are composed of two or more silicate layers; adjacent silicate layers within such particles maintain a distance of 10 ,~ from the centres of their octahedral sheets. Fundamental particles of smectite are composed of a single silicate layer. Fundamental particles of illite and smectite are capable of sorbing a monolayer of ethylene 9 1990 The Mineralogical Society