Quantitative mineralogical analysis of clay-bearing rocks is often a non-trivial problem because clay minerals are characterized by complex structures and are often affected by structural disorder, layer-stacking disorder, and interstratification. In the present study, internal-standard Rietveld X-ray powder diffraction (XRPD) analyses were combined with X-ray fluorescence (XRF) chemical analyses for the mineralogical characterization and quantitative analysis of heterogeneous clay-rich sedimentary rocks that are involved in a slow-moving landslide in the Termini-Nerano area, Sorrento Peninsula (Italy), in order to investigate the relationship between the mineralogy of these rocks and landslides. Slow-moving landslides are usually considered to be associated with the more weathered and surficial parts of structurally complex slopes, and mineralogical analysis can help to clarify the degree of weathering of siliciclastic rocks. XRPD quantitative analyses were conducted by combining the Rietveld and internal standard methods in order to calculate the amounts of poorly ordered phyllosilicate clays (considered amorphous phases in Rietveld refinements) by difference from 100%. The vbAffina program was used to refine the amounts of mineral phases determined with XRPD using the element compositions determined by XRF analysis. XRPD analyses indicated that the samples mainly contain several different clay minerals, quartz, mica, and feldspars. Analysis of the clay fraction identified kaolinite, chlorite, and interstratified illite-smectite (I-S) and chlorite-smectite (C-S). The mineralogy of the materials involved in the landslide in comparison with the mineralogy of the “undisturbed” rocks showed that the landslide is located in the weathered realm that overlies an arkosic bedrock. The interstratified I-S and C-S occurred at landslide activity locations and confirmed that areas more susceptible to sliding contained the most weathered parts of the rocks and perhaps represent areas of past and currently active fluid flow.